Add the GitHub Pages site and the docs deploy workflow

[QuasarAppCI]

Resolves #13

Description

This PR adds the GitHub Pages site of the EasySSL repository (the pattern is the same as the other QuasarApp projects, for example QtBigInt):

• pages/index.html - the EasySSL landing page: the hero with the project description, the feature list, the build options, quick usage and CMake build examples and the link to the documentation.
• doc/ - the full API reference generated with Doxygen from the sources and the README.md (the main page). It is linked from the landing page.
• .github/workflows/deploy-docs.yml - the Actions workflow that is run on every push to main (and manually via workflow_dispatch):
  1. checks out the repository with the full history (needed for the version calculation) and the submodules (the Doxygen style sheet is in the CMake submodule),
  2. installs Doxygen and Graphviz with the doxygen/setup-doxygen@v2 marketplace action,
  3. generates the doxygen.conf from doxygen.conf.in (the project version is 0..),
  4. builds the HTML documentation,
  5. stages the site (the landing page in index.html and the documentation in the doc/ folder) and deploys it to the gh-pages branch with the JamesIves/github-pages-deploy-action@v4 marketplace action.

Only the marketplace actions are used (no custom or self-hosted steps).

Documentation updates (the comment from the issue)

• README.md - added the qmake build example, the BUILD_WITH_OPENSSL build option (build the OpenSSL 3.0+ library together with EasySSL; the supported platforms are now Linux, Windows (MinGW), Android, iOS and macOS - the stale 'Windows is not supported' note is removed) and the EASYSSL_TESTS option. The links to the documentation now point to the GitHub Pages site.
• docs/USAGE.md - updated the supported platforms of the BUILD_WITH_OPENSSL build (the Windows MinGW platform is now supported, see EASYSSL_STATIC_SSL ? #6 and Resolve Issue #12: Add GitHub Actions CI workflow for Linux and Windows #14) and the link to the full doxygen documentation.

Notes

• The site will be available on https://quasarapp.github.io/easyssl/ after the merge into main (the repository settings must have the 'Deploy from a branch' source and the gh-pages branch selected in Settings > Pages).
• The doxygen.conf file is generated in the workflow and is in the .gitignore (the checked-in copy is only a local artifact).