Add a note about setting the stable API version

[b1ron]

Description

I am unsure if we should use more restrictive wording here because we don't know the extent of this issue.

Readiness checklist

• I added/updated unit tests (and they pass).
• I added/updated integration/compatibility tests (and they pass).
• I added/updated comments and checked rendering.
• I made spot refactorings.
• I updated user documentation.
• I ran task all, and it passed.
• I ensured that PR title is good enough for the changelog.
• (for maintainers only) I set Reviewers (@FerretDB/core), Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

[codecov]

Codecov Report

Merging #3035 (b6d89b2) into main (2540a9f) will decrease coverage by 0.09%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3035      +/-   ##
==========================================
- Coverage   76.50%   76.42%   -0.09%     
==========================================
  Files         386      386              
  Lines       21120    21120              
==========================================
- Hits        16157    16140      -17     
- Misses       4044     4058      +14     
- Partials      919      922       +3     

see 5 files with indirect coverage changes

Flag	Coverage Δ
hana	?
integration	72.92% <ø> (-0.10%)	⬇️
mongodb	5.41% <ø> (ø)
pg	66.32% <ø> (-0.04%)	⬇️
shard-1	54.21% <ø> (-0.10%)	⬇️
shard-2	55.76% <ø> (-0.07%)	⬇️
shard-3	57.10% <ø> (ø)
sqlite	46.53% <ø> (-0.07%)	⬇️
unit	24.25% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

[Fashander]

LGTM. Left a suggestion on the "note". Docusaurus has a built-in way for it.

[AlekSi]

LGTM, but CI fails, probably due to trailing whitespace. Please run task docs-fmt and push results.