Improve documentation about GitHub label management

[codificat]

Problem statement

As a contributor to one of the Thoth GitHub repos, it is unclear to me how the labels that are available in each repo are maintained.

The current reference in the
README
is not sufficient. It does not explain:

• when does prow update the labels
• how/if a manual update to the labels can be requested

High-level Goals

Explain the workflow to maintain the repo labels in more detail:

• clarify the relationship between labels.yaml, labels.md, and the actual labels available in GitHub
• the prow configuration that checks and implements the changes
• how to trigger a label update in the repos

Additional context

This need arose after the recent addition of a new repository in the org: https://github.com/thoth-station/thoth-github-action

That repository did not get the labels configured, and it is unclear from the current documents why this did not happen and how to fix it.

Acceptance Criteria

Documentation exists about:

• when/how are labels updated in a GitHub repo
• how to manually trigger updates

[codificat]

/sig user-experience
/triage accepted
/assign

[goern]

/kind feature