Add documentation about sites-ecmwf.int support

[Ozaq]

Description

Adds comprehensive documentation for publishing to sites.ecmwf.int, covering the full GitHub Actions-based infrastructure: reusable workflows (PR preview publish/unpublish, versioned docs publish), composite actions (sites upload, version selector generation, PR description update), architecture diagram, site layout, and copy-pasteable YAML examples.

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.