[WWW/DOC] CI: build only parts that changed.

[cederom]

• current CI process rebuilds whole website including documentation for older releases in one process that eats up unnecessary time and CI resources.
• improve website CI build process to rebuild only affected parts - for instance quick rebuild only upstream/master documentation.
• preserve option to build everything in case it is necessary (i.e. testing old releases build, or full rebuild required) - this can be launched once per week and not on every change.
• when change detection works attach "current" nuttx documentation rebuild to nuttx / master so website is in sync with repo.