Motivation

With #25 a new workflow was introduced to make sure that we tag all our PRs if they should trigger an update to the documentation.
Now we want to scale this to explicitly declare if a PR should be mentioned in the release highlights / release notes.
This PR introduces a new type of labels: "notes: needed" and "notes: skip", and cleans up / restructures some of the existing label handling.
A new workflow is introduced which checks all the three different types: semver, notes, and docs.

Changes

• Rename the docs labels from skip-docs and needs-docs to docs: skip and docs: needed to be in line with the format of the other labels (aws: <service>, semver: <type>,...). In my opinion this also has a positive impact on the UX (the search results for the label categories are easier to read).
• Add the new label category "notes": notes: skip and notes: needed.
• Adds a new workflow which checks all the labels on PRs: semver, docs, and notes.
• Fixes some copy pasta with names of existing workflows.
• Updates / fixes / cleans up the README.

TODO / Follow Ups

• Discuss if the naming changes are accepted among the org.
• Migrate all repos to use the new combined workflow instead of the existing ones.
• Remove the separate workflows (pr-enforce-semver-labels, pr-enforce-docs-labels, pr-enforce-pr-labels) if they are not used anymore / nowhere else.