docs: migrate flutter extension

[medubelko]

• Have you followed the guidelines for contributing?
• Have you signed the CLA?
• Have you successfully run tox run -m lint?
• Have you successfully run tox run -e test-py310? (supported versions: py39, py310, py311, py312)

---

Like before, please ignore the base files and only review docs/reference/extensions/flutter-extension.rst.

https://canonical-snapcraft--5226.com.readthedocs.build/en/5226/reference/extensions/flutter-extension/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.68%. Comparing base (654871d) to head (623f7d0).
Report is 684 commits behind head on main.

❗ There is a different number of reports uploaded between BASE (654871d) and HEAD (623f7d0). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (654871d)	HEAD (623f7d0)
	2	1

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5226      +/-   ##
==========================================
- Coverage   94.88%   89.68%   -5.21%     
==========================================
  Files         658      342     -316     
  Lines       55189    22636   -32553     
==========================================
- Hits        52364    20300   -32064     
+ Misses       2825     2336     -489     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[mr-cal]

The content looks good, but I added onto Alex's comment about the merit of documenting an extension that can't be used in Snapcraft 8.

I can't think of a good alternative, since Snapcraft 7 doesn't have RTD.

[lengau]

Looks great, thanks!