📝 Add "How To" instructions to README (#12)

The "How To" instructions tell you how to add another libhal package to
the API docs.

Author: kammce

README.md

@@ -1,2 +1,154 @@
 # api
-Stores and makes available API docs for all libhal packages
+
+Stores and makes available API docs for all libhal packages and their various
+versions.
+
+## 📂 File Layout
+
+```plaintext
+.
+├── LICENSE
+├── README.md
+├── libhal
+│   ├── 4.10.0
+│   ├── main
+│   └── switcher.json
+├── <package>
+│   ├── <version 1>
+│   ├── <version 2>
+│   └── switcher.json
+└── libhal-util
+    ├── 5.4.3
+    ├── main
+    └── switcher.json
+```
+
+Each directory corresponds to package or repo in the organization. The
+subdirectories are named for each version (or branch) that API docs are made
+for. Contained within are the static HTML, JS, and CSS files generated from
+Doxygen and Sphinx. The `switcher.json` file is used to provide a mapping from
+"version" to a URL which will appear on a drop down in the rendered
+documentation. The URL for the switcher MUST be absolute and is hard coded at
+documentation build time.
+
+## 🚀 Adding API doc support for a package
+
+### ⚙️ Deploy API Action
+
+> [!NOTE]
+> This is for libhal project only. At some later time I will document how
+> general C++ projects can use this same setup to deploy API docs for
+> themselves.
+
+Add the following file to your github workflow `/.github/workflow/api.yml`:
+
+```yaml
+name: 📚 Deploy APIs
+
+on:
+  release:
+    types:
+      - published
+  push:
+    branches:
+      - main
+  workflow_dispatch:
+
+permissions:
+  contents: write
+
+jobs:
+  deploy_api_docs:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - uses: actions/setup-python@v4
+        with:
+          python-version: 3.x
+      - run: sudo apt update
+      - run: sudo apt install -y doxygen
+      - run: pip install -r docs/requirements.txt gitpython requests
+      - run: wget https://raw.githubusercontent.com/libhal/ci/5.x.y/scripts/api.py
+      - run: python api.py build --version ${{ github.ref_name }}
+      - run: python api.py deploy --version ${{ github.ref_name }} --repo-name ${{ github.event.repository.name }}
+        env:
+          GITHUB_TOKEN: ${{ secrets.API_PUSH_TOKEN }}
+```
+
+> [!info]
+> Notice the `api.py` file. This file is the program that makes all of the
+> magic happen. The script above downloads the latest `5.x.y` version of the
+> script from the `libhal/ci` directory in order to build and deploy the API
+> docs. If there is something wrong, its likely to have to do with the `api.py`
+> script.
+
+### 🔑 Generating PAT token
+
+Notice the line `GITHUB_TOKEN: ${{ secrets.API_PUSH_TOKEN }}`. This is a PAT
+(Personal API Token). The default `GITHUB_TOKEN` only provides limited access
+to other repos. You cannot push to repos within an organization without a token
+with higher permissions. You need a token that has the permissions to update
+this repo and we'd like to limit that token to only being capable of updating
+this repo.
+
+1. To generate a PAT token follow the instructions in this page:
+[🔗 Creating a fine-grained personal access token](https://docs.github.com/en/authentication/keeping-your-account-and-data-secure/managing-your-personal-access-tokens#creating-a-fine-grained-personal-access-token).
+1. When you get to the point where you can choose the **Repository access**
+   Select: `Only select repositories`.
+   1. In the dropdown select: Select `<organization>/api`
+1. Set **Repository Permissions** access to:
+   1. Set `Contents` ➡️ Access: "Read and Write"
+   1. Set `Pull Requests` ➡️ Access: "Read and Write"
+1. Press **Save**
+1. You should be presented with a token, copy the token value and paste to a
+   text editor for safe keeping.
+1. Go to your organization's secrets page using the URL:
+   `https://github.com/organizations/<organization>/settings/secrets/actions`.
+1. Click the button `new organization secret`
+1. Set the name to `API_PUSH_TOKEN` and the value to the token you copied
+   earlier. If you have no further use for the token, you can delete from your
+   text editor.
+
+### 📑 Copy `docs/` from `libhal`
+
+The contents of `libhal`'s `docs/` directory has all of the doxygen and sphinx
+files needed to generate the API docs like `libhal`. Our docs use doxygen to
+generate XML docs based on the C++ code in the `include/` (the public API)
+directory. This is fed into `Sphinx` using the `Breathe` plugin to combine
+`doxygen` with `Sphinx`. The theme we use is the
+[`PyData` theme](https://pydata-sphinx-theme.readthedocs.io/en/stable/).
+
+Within the `docs/libhal` directory you will find `.md` files and a `index.rst`
+file. The `index.rst` file organizes all of the markdown files and the markdown
+files are used to manually type out how you want the docs for a particular class
+or source file to be rendered. Change these to fit your package.
+
+Update the `json_url` for the switcher to the following and update the
+`<organization>` and `<package>` templates to the correct value:
+
+```python
+html_theme_options = {
+    "switcher": {
+        "json_url": "https://<organization>.github.io/<organization>/<package>/switcher.json",
+    },
+}
+```
+
+Update these lines and replace `<package>` with the name of your package:
+
+```python
+breathe_projects = {"<package>": "../build/xml"}
+breathe_default_project = "<package>"
+breathe_default_members = ('members',)
+project = "<package>"
+```
+
+Update `docs/index.rst` to fit what you'd like to show as the front page of your
+package.
+
+### 🎉 Done
+
+And you are done! The next time you push to main, make a release, OR manually
+run the `api.yml` workflow, your API docs will be generated, a new branch,
+commit, and PR will be made to `organization/api` that you can later decide to
+pull into the repo or decline.