Revamp labels documentation

What does this MR do?

A full revamp of the labels documentation, including:

• substantial simplifications and clarifications
• rephrasing to avoid the use of screenshots as much as possible
• deleting unused screenshots, and cropping one unnecessarily large one
• restructuring the ordering of the sections, to simplify and group related things together.
• removing sections that are duplicated in other pages (searching/filtering)
• adding crosslinks to other docs when possible/needed

Related issues

Related to #199399 (closed)

Followup

Verify that no links/anchors (to /help) were broken in the product UI itself.

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• If applicable, update the permissions table.
• Link docs to and from the higher-level index page, plus other related docs where helpful.
• Apply the documentation label.

Review checklist

All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.

1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

2. Technical Writer

• Optional: Technical writer review. If not requested for this MR, must be scheduled post-merge. To request for this MR, assign the writer listed for the applicable DevOps stage.

3. Maintainer

1. Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
2. Ensure a release milestone is set.
3. If there has not been a technical writer review, create an issue for one using the Doc Review template.