Reorg and update attributes

[ehuss]

• Move "Attributes" out of the "Items" chapter.
• Move each attribute out of attributes.md, scattered to various locations throughout the reference, including a few new dedicated pages. Each attribute gets a dedicated section to make it easier to link to, and enough room to fully document.
• See attributes.md for the consolidated built-in index.
• Document the syntax for the inputs of every built-in attribute.
• Minor updates and additions to various attributes.
• Removed no_start, it is no longer used. Removed when runtime/green threads were removed Finish runtime removal rust#18967.

Things reserved for future PRs:

• Expanding individual attributes. Some are too brief or slightly wrong.
• Documenting the remaining undocumented attributes.
• Finish the "Attributes on" sections to specify where each attribute may be used.

Some of the new locations seem a little awkward or arbitrary, but I couldn't think of better choices. I wanted to open this up to start getting feedback.

This is written assuming rust-lang/rust#58899 will get merged. Also, some attributes are not very strict about validating their inputs (for example, link ignores any unknown field). I documented with what I think they are intended, not what they actually accept.

Closes #534