Remove separate docs GitHub workflow

[findepi]

Before the change, we had two workflows: ci and docs.

• for code changes we run ci
• for docs changes we run docs
• for code and docs changes we run both, but the docs PR was configured
  to do nothing

ci has build-success status marker and it's required job for a PR to
be merged.

We could add similar job to docs workflow.

• it can also be called build-success. This would work for docs-only
  PRs, but not for code & docs PRs.
• it can be called docs-build-success and also become required, but
  that doesn't solve original problem at all.

Solving this requires that

• the workflows to be truly mutually exclusive (for now they are
  logically mutually exclusive but not really) OR
• there is only one workflow, with docs workflow merging into ci
  workflow.

This change implements the latter option, as it's conceptually simpler
and so should be better from long term maintainability perspective.

• The docs workflow is removed.
• The ci workflow is run on all PRs.
• The docs-checks is already covered by maven-checks job in ci
  workflow.

The removed docs workflow had one more job: test-docs. This was
supposed to run Java tests that check docs changes. This is currently
lost. However, it wasn't working reliably anyway. It would run on
docs-only changes, but would not run on PRs mixing docs changes with any
other code changes, unless those changes would trigger these tests to
run (#27076). This is left to be fixed later.

• fixes Docs-only or README-changing PRs cannot be merged #27068

Summary by Sourcery

Merge the separate docs GitHub workflow into the main CI workflow by introducing path-based filtering, reintroducing a final build-success job, and removing docs.yml.

Enhancements:

• Update build-matrix-from-impacted.py to run all modules when the impacted list is empty

Build:

• Update docs/pom.xml to run code generation in the generate-sources phase instead of validate
• Modify cleanup.yml to cancel only the ci.yml workflow

CI:

• Merge docs.yml into ci.yml and remove the standalone docs workflow
• Introduce a path-filters job to classify docs vs non-docs changes and conditionally skip CI jobs for docs-only PRs
• Reinstate the build-success job as the final aggregator of all CI job results

Tests:

• Refactor TestCiWorkflow to use a BUILD_SUCCESS constant and add a test ensuring the build-success job is last in ci.yml

[findepi]

The workflow changes were tested here

• Test docs-only change on CI #27066
• Test docs and code change on CI #27067
• Test code-only change on CI #27071
• Test readme change on CI #27073
• Test breaking docs on CI #27074

[electrum]

Will build the docs module if the dependent Java module changes? Specifically, if you add another reserved word for the parser, does the ReservedIdentifiers check still run?

[findepi]

Will build the docs module if the dependent Java module changes? Specifically, if you add another reserved word for the parser, does the ReservedIdentifiers check still run?

Yes

• a build against current master Test docs build for new reserved identifier (master) #27085
• a build against this PR Test docs build for new reserved identifier (docs workflow changes) #27086

Before and after the changes, this is covered by ci workflow, the maven-checks job.
The docs workflow (the one being removed here) does nothing for a PR that has any non-docs changes.

[findepi]

CI

• setup action sometimes fails in Clearing up disk usage step #26943
• Flaky test TestAlluxioFileSystem and TestAlluxioFileSystemExchangeManager #23596

i am going to squash fixups now