CASSJAVA-109, CASSJAVA-117: Manual and API References after donation, Build CI for Java Driver Doc:

[SiyaoIsHiding]

Several things this PR do:

1. Fix the broken Java doc and broken links in the existing manual
2. mkdocs config e..g mkdocs.yaml, folder docs of symbol links as mkdocs' working directory, and the build script "./build-doc.sh"
3. GH Action CI to upload built doc as an artifact on PR e.g. Download the zip here. Unzip, and run this command in the root directory to preview the website: python -m http.server (with python3).
4. GH Action to update the branch gh-pages-staging on 4.x commit (PR merged). Example run: https://github.com/SiyaoIsHiding/java-driver/actions/runs/19521378245 (someone with write access need to create that branch first)