186 improve examples ci workflow

[paweljarosz]

This PR changes a CI in this repo:

• On PR, .github/workflows/validate-examples.yml runs only if relevant paths changed.
• In that PR workflow, it validates only touched example.md files and builds only the touched example projects with Bob using the latest beta version.
• On push to master, .github/workflows/trigger-site-rebuild.yml again validates only touched example.md files and builds only touched example projects.

This is to ensure the following flow:

1. On creating a PR: validate + build touched examples.
2. On merge to master: validate + build touched examples again.
3. Only then: trigger defold.github.io site rebuild.

To make the last work we need to add this validation and build steps as required after merging, somewhere here in the Github page settings.

This should be merged after: defold/defold.github.io#253

#186