Add documentation for "directives" language feature

[btiernay]

Clear and concise description of the problem

Currently, there is no documentation section that describes this part of the language, including its syntax and semantics. However, there are several references to it and backing APIs which support it:

• https://typespec.io/docs

• https://typespec.io/docs/standard-library/built-in-decorators

• https://typespec.io/docs/next/standard-library/reference/js-api/type-aliases/Directive

Checklist

• Follow our Code of Conduct
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.