Add docs for migrating to MongoDB from FerretDB

[Fashander]

Description

Closes #3229

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 #3374 (53c5d76) into main (33cdbc4) will decrease coverage by 20.11%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main   #3374       +/-   ##
==========================================
- Coverage   24.87%   4.76%   -20.11%     
==========================================
  Files         414     408        -6     
  Lines       25356   25040      -316     
==========================================
- Hits         6307    1194     -5113     
- Misses      18449   23762     +5313     
+ Partials      600      84      -516     

Flag	Coverage Δ
integration	4.76% <ø> (ø)
mongodb-1	4.76% <ø> (ø)
pushdown-true	4.76% <ø> (ø)
unit	?

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

[AlekSi]

#3374 (review)

[AlekSi]

Added not ready to stop Mergify