Add a "starting a new goal section" documentation

[tomassedovic]

We don't have any documentation on how to start a new Goals period (e.g. "2026").

Here's an incomplete list of what needs to happen:

• Figure out which existing goals we'll move forward
• Create the new GitHub label for the goal tracking issues (e.g. )
• Create a new GitHub milestone (e.g. 2026)
• Create the goal subdirectory under src with:
  • this is where the goals will live
• Add the new section (e..g 2026 goal process) to src/SUMMARY.md and fill in the files this section links to
• Set an end date for the current period reports (e.g. replace (((REPORTS: 2025-09-01 to 2025-12-31))) with (((REPORTS: 2025-09-01 to 2025-12-31))) to 2026-03-31 in src/2025h2/reports.md)
• Set the new period's report to an open-ended date with the goal start (e.g. (((REPORTS: 2026-04-01))))
• Update the template if we've made changes: https://github.com/rust-lang/rust-project-goals/blob/main/src/TEMPLATE.md
• Open the RFC (e.g. propose 2025h2 Goals rfcs#3849)
• Update src/README.md with the RFC text
• Move goals we want to continue to the new milestone and close the ones we won't (Niko has a script for this apparently)

In particular I wanted to highlight the start with an optional end date REPORTS range as a relatively new thing that we're not really tracking anywhere other than the source code yet (#441 (comment)).