Add blog post on Adding MongoDB compatibility to Aiven for PostgreSQL

[Fashander]

Description

Closes FerretDB/engineering#173.

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.

[mergify]

Marketing blog posts should be reviewed by @ptrfarkas and @AlekSi.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.75%. Comparing base (453e731) to head (bf7f9a2).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4349      +/-   ##
==========================================
- Coverage   75.25%   74.75%   -0.50%     
==========================================
  Files         326      326              
  Lines       22471    22471              
==========================================
- Hits        16910    16799     -111     
- Misses       4339     4437      +98     
- Partials     1222     1235      +13     

see 22 files with indirect coverage changes

Flag	Coverage Δ
filter-false	?
filter-true	68.19% <ø> (-0.77%)	⬇️
hana-1	13.96% <ø> (ø)
integration	68.19% <ø> (-0.80%)	⬇️
mongodb-1	5.10% <ø> (ø)
mysql-1	?
mysql-2	?
mysql-3	?
postgresql-1	44.64% <ø> (-8.50%)	⬇️
postgresql-2	44.79% <ø> (-9.62%)	⬇️
postgresql-3	41.51% <ø> (-12.02%)	⬇️
postgresql-4	41.06% <ø> (-0.01%)	⬇️
postgresql-5	43.25% <ø> (-0.05%)	⬇️
sqlite-1	43.65% <ø> (-8.45%)	⬇️
sqlite-2	44.11% <ø> (-9.30%)	⬇️
sqlite-3	40.72% <ø> (-11.95%)	⬇️
sqlite-4	40.42% <ø> (-0.08%)	⬇️
sqlite-5	42.38% <ø> (-0.01%)	⬇️
unit	33.11% <ø> (ø)

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