[FLINK-39237][Doc] Decouple flink-training links from the Flink branch

[joeyutong]

What is the purpose of the change

This fixes broken flink-training links in the docs on release branches.

Today the training_link shortcode uses Site.Params.Branch, which is updated to values such as release-2.2 when a new Flink docs branch is created. The apache/flink-training repository does not create matching release branches, so those links resolve to non-existent GitHub paths and return 404s.

Brief change log

This change introduces a dedicated TrainingExercisesBranch config entry and updates the training shortcodes to use it instead of the Flink docs Branch.

• add TrainingExercisesBranch = "master" to docs/config.toml
• update training_link.html to use TrainingExercisesBranch
• update training_repo.html to use TrainingExercisesBranch

Verifying this change

Build and verify the docs in local.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

[flinkbot]

CI report:

• 7165b87 Azure: SUCCESS

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot run azure re-run the last Azure build