rename "Macros By Example" section to "Declarative Macros"

[lolbinarycat]

this terminology is rarely used outside of the reference, and is fairly confusing. it is especially easy to confuse this page for the rust by example page on macros.

proposed changes:

• change https://doc.rust-lang.org/reference/macros-by-example.html to https://doc.rust-lang.org/reference/declarative-macros.html
• add a redirect from the old url to the new url
• change the page title to "Declarative Macros"
• add "sometimes called Macros By Example" to the page body.