Add blogpost for release v.1.15

[Fashander]

Description

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

Merging #3728 (fd02d6b) into main (c1dbb63) will decrease coverage by 0.03%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3728      +/-   ##
==========================================
- Coverage   76.51%   76.49%   -0.03%     
==========================================
  Files         323      323              
  Lines       23070    23070              
==========================================
- Hits        17653    17647       -6     
- Misses       4382     4386       +4     
- Partials     1035     1037       +2     

see 2 files with indirect coverage changes

Flag	Coverage Δ
filter-true	72.63% <ø> (-0.03%)	⬇️
hana-1	3.52% <ø> (ø)
integration	72.63% <ø> (-0.03%)	⬇️
mongodb-1	5.45% <ø> (ø)
postgresql-1	51.82% <ø> (-0.01%)	⬇️
postgresql-2	51.46% <ø> (ø)
postgresql-3	52.62% <ø> (+0.05%)	⬆️
sort-false	72.63% <ø> (-0.03%)	⬇️
sqlite-1	50.98% <ø> (ø)
sqlite-2	50.62% <ø> (ø)
sqlite-3	51.80% <ø> (ø)
unit	30.17% <ø> (ø)

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