[QuasarAppCI]

Resolves #17

Description

The "Deploy Documentation (GitHub Pages)" workflow (added by #16) fails on main:
the install step references the action doxygen/setup-doxygen@v2, but that
repository does not exist on GitHub, so the run stops at the action resolution
("Unable to resolve action doxygen/setup-doxygen, repository not found",
run 38308295589).

This PR replaces the install step with real marketplace actions, as requested in
#13 (marketplace actions for the dependencies):

• ssciwr/doxygen-install@v2.0.3 - installs Doxygen from the official
  releases, pinned to 1.9.8 (the version the documentation was generated
  and validated with);
• ts-graphviz/setup-graphviz@v2.0.2 - installs the Graphviz dot tool
  (the official Graphviz action; the Doxygen config has HAVE_DOT = YES, so
  the class diagrams require it - and it is not preinstalled on the
  ubuntu-latest runner).

The rest of the workflow (Doxygen config generation, the HTML build, the site
staging and the JamesIves/github-pages-deploy-action@v4 deploy to the
gh-pages branch) is unchanged.

Validation

• Simulated the workflow steps locally with Doxygen 1.9.8 + Graphviz:
  doxygen doxygen.conf exits 0, the class diagrams are rendered (dot PNGs
  generated) and the staged site has the expected layout
  (stage/index.html + stage/doc/ with the extra stylesheet).
• The "Build and Test" CI runs on this PR as well.

After the merge the workflow is re-triggered by the push to main and the site
is deployed to https://quasarapp.github.io/easyssl/.

[QuasarAppCI]

CI is green (Build and Test: ubuntu-latest + windows-latest SUCCESS). Ready for review - main requires an approving review from a reviewer with write access before the merge.

The change is limited to the two install steps of the workflow; the Doxygen build itself was re-validated locally with Doxygen 1.9.8 + Graphviz (exit 0, class diagrams rendered, staged site = index.html + doc/ with the extra stylesheet).

[EndrII]

good job