simple-repository-api: clean up, add API history

[woodruffw]

This follows #1803, and performs a few cleanup actions on the simple repository API docs:

• I've removed the Additional Fields for the Simple API for Package Indexes section entirely and have folded its prose into the relevant parts of the JSON Serialization section. This makes the doc as a whole easier to follow, since the user doesn't need to tab down to a different adjacent section to learn about the changes made with API version 1.1. It's also consistent with that was done with API version 1.3, i.e. PEP 740.
• I've added an API Version History subsection that lists only API versions and the PEPs that stabilized them. This should make it easier to cross-reference changes.
• I've moved the standard RFC 2119 language to the top of the document, since the entire doc uses it and not just the subsection it previously was in.
• I've added a few instances of proper HTML syntax highlighting.

CC @di @brettcannon @nlhkabu

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📚 Documentation preview 📚: https://python-packaging-user-guide--1808.org.readthedocs.build/en/1808/

[willingc]

@woodruffw Thanks for these helpful changes. The prose and rendering look good to me.