Add documentation for the #deprecated directive

[scarletcs]

The documentation for @deprecated is deprecated, as noted on its page, per issue #2214, and says the following on its page:

Deprecated: @deprecated decorator is deprecated. Use the #deprecated directive instead.

That much is all fine.

However, there is no documentation on the #deprecated directive.

• The instruction to use the #deprecated directive does not link to documentation for the new feature. (It should!)
• There is no page titled #deprecated.
• There is no page titled Directives, or dedicated to explaining what directives are and which directives exist.
• There is no entry for Directives in the Language Basics table of contents. Its overview page also does not mention directives.

The example for @deprecated says to use the deprecated directive and provides the only example I can find of using it, but this is not exactly primary documentation for #deprecated or for what a directive is or what makes it different to a decorator.