Document createIndexes, listIndexes, and dropIndexes commands

[Fashander]

Description

Closes #2271.

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

[codecov]

Codecov Report

Merging #2488 (5953472) into main (27fc560) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2488   +/-   ##
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  Coverage   65.15%   65.15%           
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  Files         410      410           
  Lines       20071    20071           
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  Hits        13077    13077           
  Misses       6055     6055           
  Partials      939      939           

see 1 file with indirect coverage changes

Flag	Coverage Δ
integration	57.89% <ø> (ø)
mongodb	5.04% <ø> (ø)
pg	57.82% <ø> (ø)
unit	25.69% <ø> (ø)

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

[rumyantseva]

Suggested a few things and asked a few questions.

There is one thing to clarify: commands and methods/wrappers/handers are not the same.
Commands are on the server (FerretDB) side, and methods/wrappers/handers are on the client/driver side.
There could be a few methods to call the same command. For example, both dropIndex() and dropIndexes() methods call dropIndexes() command.

I find this mixture of methods and commands a bit inconsistent, but probably this is how we use in the other parts of our docs too...

[rumyantseva]

LGTM

[Fashander]

@AlekSi PTAL