Add mermaid diagrams

[AlekSi]

Description

https://github.com/AlekSi/FerretDB/blob/mermaid/README.md

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 #3524 (3540361) into main (35f5849) will decrease coverage by 0.15%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #3524      +/-   ##
==========================================
- Coverage   75.05%   74.91%   -0.15%     
==========================================
  Files         427      427              
  Lines       27119    27119              
==========================================
- Hits        20354    20315      -39     
- Misses       5556     5596      +40     
+ Partials     1209     1208       -1     

see 13 files with indirect coverage changes

Flag	Coverage Δ
filter-true	71.75% <ø> (-0.22%)	⬇️
hana-1	?
integration	71.75% <ø> (-0.22%)	⬇️
mongodb-1	4.50% <ø> (ø)
pg-1	38.95% <ø> (ø)
pg-2	41.93% <ø> (+0.03%)	⬆️
pg-3	42.39% <ø> (ø)
postgresql-1	38.96% <ø> (-0.05%)	⬇️
postgresql-2	?
postgresql-3	42.42% <ø> (-0.23%)	⬇️
sort-false	71.75% <ø> (-0.22%)	⬇️
sqlite-1	38.15% <ø> (+0.02%)	⬆️
sqlite-2	41.33% <ø> (ø)
sqlite-3	41.84% <ø> (-0.01%)	⬇️
unit	25.97% <ø> (+0.01%)	⬆️

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