Move Optimization Detective docs into README.md

[westonruter]

After the most recent release of Optimization Detective, I discovered that the plugin directory has a 2,500 word limit for readme.txt files. And Optimization Detective now exceeds the limit:

This is resulting in the readme being truncated:

So this PR moves the Background, Technical Foundation, Use Cases/Examples, and Hooks into a README.md which is then linked to from the readme.txt. The initial two paragraphs of the plugin description are retained in both places for context. Sections are also added for installation, feedback, support, contributing, and security. There is also a new section added for extension plugins.

This also further improves the OD-related plugin readmes with a notice that the REST API must be available to unauthenticated visitors. See #1731.

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[westonruter]

• The formatting of the accepted arguments to each action/filter could be added and improved.

[westonruter]

Also good to add, although not necessary immediately:

• How to write an extension
• Class reference
• Function reference
• JSON Schema

[felixarntz]

Maybe we should add a dedicated docs folder, or a Wiki, or something like that? The documentation needs for OD would probably justify a dedicated location like that.

[westonruter]

I thought about a Wiki too, but @joemcgill suggested a README due to it being easier to keep up to date in code (as changes can land with new features in PRs). At the same time, edits are also burdensome since they have to go through the PR process. In any case, I think a README as a landing page makes sense and then we can break out sections into separate docs pages, whether that be in a docs subdirectory or a Wiki (which we'd have to enable for the repo, but then why not use the Handbook?)