Update domain name in docs

[AlekSi]

Description

ferretdb.io -> ferretdb.com

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 #3757 (06f0b79) into main (aa05f0f) will decrease coverage by 0.16%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3757      +/-   ##
==========================================
- Coverage   76.62%   76.47%   -0.16%     
==========================================
  Files         324      324              
  Lines       23090    23090              
==========================================
- Hits        17693    17658      -35     
- Misses       4369     4398      +29     
- Partials     1028     1034       +6     

see 9 files with indirect coverage changes

Flag	Coverage Δ
filter-false	?
filter-true	72.52% <ø> (-0.09%)	⬇️
hana-1	?
integration	72.52% <ø> (-0.16%)	⬇️
mongodb-1	5.48% <ø> (ø)
postgresql-1	51.80% <ø> (-0.15%)	⬇️
postgresql-2	51.45% <ø> (-0.08%)	⬇️
postgresql-3	52.50% <ø> (-0.22%)	⬇️
sqlite-1	50.95% <ø> (-0.05%)	⬇️
sqlite-2	50.61% <ø> (-0.03%)	⬇️
sqlite-3	51.80% <ø> (-0.04%)	⬇️
unit	30.31% <ø> (ø)

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