docs: clarify flag availability across controller binaries

[lusqua]

Changes

Restructure the docs/tekton-controller-flags.md documentation to clearly communicate which flags and environment variables are available on each controller binary (controller, webhook, events, resolvers).

What changed

• Separated flags by source: flags from knative/pkg (common to all controllers) are now clearly distinguished from Tekton-specific flags
• Added per-controller availability table: a table shows exactly which flags and env vars each controller supports
• Added per-controller sections: each controller now has its own section documenting its specific flags and environment variables
• Corrected inaccuracies: source code verification revealed that:
  • resolvers does not support --disable-ha (it uses MainWithConfig, not MainWithContext)
  • controller and resolvers use THREADS_PER_CONTROLLER env var (no K_ prefix), while webhook and events use K_THREADS_PER_CONTROLLER (with K_ prefix) via sharedmain

Fixes #9292

Submitter Checklist

As the author of this PR, please check off the items in this checklist:

• Has Docs if any changes are user facing, including updates to minimum requirements e.g. Kubernetes version bumps
• Has Tests included if any functionality added or changed
• pre-commit Passed
• Follows the commit message standard
• Meets the Tekton contributor standards (including functionality, content, code)
• Has a kind label. You can add one by adding a comment on this PR that contains /kind <type>. Valid types are bug, cleanup, design, documentation, feature, flake, misc, question, tep
• Release notes block below has been updated with any user facing changes (API changes, bug fixes, changes requiring upgrade notices or deprecation warnings). See some examples of good release notes.
• Release notes contains the string "action required" if the change requires additional action from users switching to the new release

Release Notes

Restructured docs/tekton-controller-flags.md to clarify flag and environment variable availability per controller binary, and corrected inaccuracies in --disable-ha and THREADS_PER_CONTROLLER documentation.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: lusqua / name: Lucas Aguiar (b3820c5)

[waveywaves]

@lusqua please sign the CLA

[waveywaves]

@lusqua also, please follow the PR template

[lusqua]

/kind documentation

[waveywaves]

/lgtm

[tekton-robot]

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