docs: cpp-tagfiles is antora extension

[alandefreitas]

The cpp extension becomes an Antora extension. This allows us to provide extra tagfiles in the playbook and component files.

[cppalliance-bot]

An automated preview of the documentation is available at https://851.url.prtest2.cppalliance.org/libs/url/doc/html/index.html

[cppalliance-bot]

GCOVR code coverage report https://851.url.prtest2.cppalliance.org/gcovr/index.html
LCOV code coverage report https://851.url.prtest2.cppalliance.org/genhtml/index.html
Coverage Diff https://851.url.prtest2.cppalliance.org/gcovr/coverage_diff.txt

[cppalliance-bot]

An automated preview of the documentation is available at https://851.url.prtest2.cppalliance.org/libs/url/doc/html/index.html

[cppalliance-bot]

Antora version: an automated preview of the documentation is available at https://851.urlantora.prtest2.cppalliance.org/site/index.html

[cppalliance-bot]

GCOVR code coverage report https://851.url.prtest2.cppalliance.org/gcovr/index.html
LCOV code coverage report https://851.url.prtest2.cppalliance.org/genhtml/index.html
Coverage Diff https://851.url.prtest2.cppalliance.org/gcovr/coverage_diff.txt

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.21%. Comparing base (eccce47) to head (bcb1ab5).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #851   +/-   ##
========================================
  Coverage    99.21%   99.21%           
========================================
  Files          157      157           
  Lines         8422     8422           
========================================
  Hits          8356     8356           
  Misses          66       66           

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update eccce47...bcb1ab5. Read the comment docs.

[cppalliance-bot]

Antora version: an automated preview of the documentation is available at https://851.urlantora.prtest2.cppalliance.org/site/index.html

[cppalliance-bot]

An automated preview of the documentation is available at https://851.url.prtest2.cppalliance.org/libs/url/doc/html/index.html

[cppalliance-bot]

GCOVR code coverage report https://851.url.prtest2.cppalliance.org/gcovr/index.html
LCOV code coverage report https://851.url.prtest2.cppalliance.org/genhtml/index.html
Coverage Diff https://851.url.prtest2.cppalliance.org/gcovr/coverage_diff.txt

[cppalliance-bot]

An automated preview of the documentation is available at https://851.url.prtest2.cppalliance.org/libs/url/doc/html/index.html

[cppalliance-bot]

Antora version: an automated preview of the documentation is available at https://851.urlantora.prtest2.cppalliance.org/site/index.html

[cppalliance-bot]

GCOVR code coverage report https://851.url.prtest2.cppalliance.org/gcovr/index.html
LCOV code coverage report https://851.url.prtest2.cppalliance.org/genhtml/index.html
Coverage Diff https://851.url.prtest2.cppalliance.org/gcovr/coverage_diff.txt