Add linter to check truncate tag in blog posts

[sbshah97]

• Added a check for the presence of the truncate string in blog post files
• Created unit tests for the new truncate string verification

Description

Closes #4048.

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.

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.76%. Comparing base (892780a) to head (c791623).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4139      +/-   ##
==========================================
- Coverage   73.78%   73.76%   -0.02%     
==========================================
  Files         353      353              
  Lines       22789    22789              
==========================================
- Hits        16814    16811       -3     
- Misses       4694     4697       +3     
  Partials     1281     1281              

see 2 files with indirect coverage changes

Flag	Coverage Δ
filter-true	67.15% <ø> (-0.07%)	⬇️
integration	67.15% <ø> (-0.07%)	⬇️
mongodb-1	4.87% <ø> (ø)
postgresql-1	48.40% <ø> (-0.03%)	⬇️
postgresql-2	47.20% <ø> (+<0.01%)	⬆️
postgresql-3	49.48% <ø> (+<0.01%)	⬆️
sqlite-1	47.58% <ø> (+0.07%)	⬆️
sqlite-2	23.34% <ø> (-23.02%)	⬇️
sqlite-3	48.64% <ø> (+0.08%)	⬆️
unit	32.49% <ø> (ø)

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

[chilagrow]

Thanks for your contribution! I updated PR title to be more specific 🤗

[chilagrow]

Thanks for your contribution!