Add query pushdown documentation

[noisersup]

Description

Closes #1956.

Readiness checklist

• I added/updated unit tests.
• I added/updated integration/compatibility tests.
• 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), Assignee, Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

[codecov]

Codecov Report

Merging #2339 (d630394) into main (6934d0f) will decrease coverage by 37.40%.
The diff coverage is n/a.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2339       +/-   ##
===========================================
- Coverage   64.27%   26.87%   -37.40%     
===========================================
  Files         392      392               
  Lines       19234    19234               
===========================================
- Hits        12362     5170     -7192     
- Misses       5969    13509     +7540     
+ Partials      903      555      -348     

see 125 files with indirect coverage changes

Flag	Coverage Δ
integration	4.73% <ø> (-52.54%)	⬇️
mongodb	4.73% <ø> (?)
pg	?
unit	25.53% <ø> (-0.03%)	⬇️

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

[rumyantseva]

I like that in this PR we aim to keep things short and link a blog post for more details.

I added a couple of comments in the places that I find a bit unclear, but feel free to ignore them if everyone else is fine with the current definitions.

[rumyantseva]

LGTM

[rumyantseva]

👍