Split CI workflow into two stages

[paweljarosz]

With a separate build stage for examples building touched examples.

The previous workflow combined generated-content updates, commits, Jekyll build, Pagefind indexing, and Pages deployment in one job. It also allowed multiple repository dispatch runs to update and push master concurrently, which made examples updates fragile and caused repeated push failures when another generated update landed first.

Now:

• Update generated site: handles repository dispatch updates and pushes generated changes.
• Build site: builds Jekyll, runs Pagefind, uploads the Pages artifact, and deploys on master.

Added workflow concurrency for generated updates so dispatch jobs that push generated content run serially.
Refreshed the checkout to latest origin/master before generating changes
Added deterministic examples imports using --examples-ref.
Added incremental examples imports using --changed-examples or --changed-examples-json.
Kept full examples import as the fallback when no changed examples payload is provided.
Made update jobs lighter by removing Jekyll/Pagefind work from the update workflow and only installing Java when needed.

Examples building is very long. This should make the update path more deterministic, reduce unnecessary examples rebuild work, and separate update failures from site build failures.

This should be merged before updates in the examples: defold/examples#187

[AGulev]

It looks good, but an idea for another task: use examples as test we can run nightly to make sure everething works and maybe even runs
e.g. building it for mac and then run and make screenshots on self-hosted ci-machine

[paweljarosz]

It looks good, but an idea for another task: use examples as test we can run nightly to make sure everething works and maybe even runs e.g. building it for mac and then run and make screenshots on self-hosted ci-machine

Yeah, great idea, I created a separate issue for it: #254