Make the writing guide accessible but unlisted

[AlekSi]

Description

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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), Milestone (Next), Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

[codecov]

Codecov Report

Merging #3221 (b8e6484) into main (31078f4) will decrease coverage by 27.23%.
The diff coverage is n/a.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #3221       +/-   ##
===========================================
- Coverage   75.46%   48.23%   -27.23%     
===========================================
  Files         396      396               
  Lines       22210    22210               
===========================================
- Hits        16761    10714     -6047     
- Misses       4464    10685     +6221     
+ Partials      985      811      -174     

see 127 files with indirect coverage changes

Flag	Coverage Δ
integration	39.01% <ø> (-32.86%)	⬇️
mongodb	5.30% <ø> (ø)
pg	?
shard-1	32.58% <ø> (-23.34%)	⬇️
shard-2	31.86% <ø> (-23.43%)	⬇️
shard-3	27.37% <ø> (-28.37%)	⬇️
sqlite	38.73% <ø> (+1.08%)	⬆️
unit	24.67% <ø> (ø)

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

[rumyantseva]

👍