Added docs for X11 key remapping (#33)

* Added docs for X11 key remapping

Lite-XL does not seem to be aware of any keyboard re-mapping done via the usual tools (such as `xkbset` or the GNOME setting).

I assume this is because Lite is built on SDL, which is getting its keyboard input via some mechanism that is bypassing the usual X11 drudgery?

Anywhoo, as a lifelong Caps -> Ctrl remapping this is kind of a big deal. Fortunately there's a pretty straightforward workaround. This is a big enough deal that I think it'll cause a lot of people to bounce off Lite-XL pretty quickly.

I'm not sure this is the best place in the docs but I think it's worth documenting *somewhere*. Happy to move locations or wording as suggested.

* Various PR feedback

* Moved everything inside a "tip" admonition
* Used the proper name "Lite XL"
* Suggest remapping via the Lite XL config

* Fix typo

* Rephrase a little bit

---------

Co-authored-by: takase1121 <20792268+takase1121@users.noreply.github.com>

Author: relaxbox

docs/user-guide/configuration.md

@@ -236,6 +236,22 @@ To remove an existing keyboard shortcut, you can use `keymap.unbind()`.
     Afterwards, press the "Save" button to save the changes.
     ![Screenshot showing the keybind changer][13]{ loading=lazy }
 
+
+!!! tip
+    On X11, Lite XL interprets keypresses based on their physical location,
+    regardless of the keyboard layout set in your DE settings.
+    Tools such as `xmodmap` will therefore have no effect; instead you'll have to modify your keymap
+    in Lite XL directly.
+
+    For example, to remap ++caps-lock++ to ++ctrl++, add this to your user module:
+
+    ```lua
+    local modkeys = require("core.modkeys-" .. (PLATFORM == "Mac OS X" and "macos" or "generic"))
+    modkeys.map["capslock"] = "ctrl" -- this maps capslock to ctrl
+    table.insert(modkeys.keys, "capslock") -- this enables capslock as modifier
+    modkeys.map["left ctrl"] = "capslock" -- this maps left ctrl to capslock
+    ```
+
 ## Themes
 
 The default theme is a dark theme.