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.

[0elek]

"Macros By Example" is not just the name of the section but the type of macros that the macro_rules! produce. i don't think this need changing since its being explained in the first line.

[lolbinarycat]

"Macros By Example" is not just the name of the section but the type of macros that the macro_rules! produce.

yes, i understand that, but that type of macro is much more commonly referred to as "declarative macros". "Macros By Example" is a confusing name, which is why most information on rust does not use it.

i don't think this need changing since its being explained in the first line.

just because something is explained in the reference doesn't mean it isn't confusing. the terminology in the reference should match that used by the rest of the community, and that is not the case here.