Improve documentation for Docker

[AlekSi]

Description

Make it as simple and accessible as I can.

Also, do several tweaks here and there.

Closes FerretDB/engineering#27.

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 #2396 (85e30ff) into main (12342c7) will decrease coverage by 37.58%.
The diff coverage is 75.00%.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2396       +/-   ##
===========================================
- Coverage   64.33%   26.75%   -37.58%     
===========================================
  Files         392      392               
  Lines       19251    19251               
===========================================
- Hits        12385     5151     -7234     
- Misses       5963    13546     +7583     
+ Partials      903      554      -349     

Impacted Files	Coverage Δ
internal/handlers/common/aggregations/stage.go	0.00% <ø> (-20.00%)	⬇️
internal/handlers/common/msg_debugerror.go	0.00% <0.00%> (ø)
internal/handlers/hana/hana.go	0.00% <ø> (ø)
internal/handlers/pg/pg.go	20.89% <ø> (-58.21%)	⬇️
internal/handlers/tigris/tigris.go	0.00% <ø> (ø)
build/version/version.go	67.64% <100.00%> (ø)

... and 123 files with indirect coverage changes

Flag	Coverage Δ
integration	4.72% <75.00%> (-52.60%)	⬇️
mongodb	4.72% <75.00%> (ø)
pg	?
unit	25.40% <75.00%> (-0.17%)	⬇️

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