Review and prune outdated docs

[colindean]

Verification

• This issue's title and/or description do not reference a single formula e.g. brew install wget. If they do, open an issue at https://github.com/Homebrew/homebrew-core/issues/new/choose instead.

Provide a detailed description of the proposed feature

The Docs team at AGM surfaced a need to review all docs (tracking enabled by @issyl0's #19200) and prune docs that are outdated, all or in part.

Notably, https://docs.brew.sh/Checksum_Deprecation could likely be dropped entirely.

What is the motivation for the feature?

Outdated docs are noisy to users who are browsing the docs for a solution. Regular review of our documentation is worthwhile to ensure current accuracy.

How will the feature be relevant to at least 90% of Homebrew users?

Homebrew users who read documentation would love an affirmation that the documentation is up to date or to know that a document may be out of date because it hasn't been reviewed in a long time.

What alternatives to the feature have been considered?

No action would leave outdated docs in place and not institute a review effort.

We consider tooling to list unreviewed docs out of scope for this.

[MikeMcQuaid]

Woohoo!

prune docs that are outdated, all or in part.

We should ensure that these docs are not linked legitimately/too widely online and not at all by our own repos/tools before doing this.