Add some signposting to the String docs for pattern matching

[DeemoONeill]

As a user of the docs, when looking for information on how to pattern match with strings and binaries the first place one looks is the String docs.

Currently there isn't anything here for pattern matching besides a small example matching a character. The user would either have to go from the reference section to the patterns and guards guide or go to the <<>>/1 special forms docs.

This change just adds a link to the special forms docs and the guide to signpost the user to those resources.

[DeemoONeill]

For reference my flow when looking for this information was

String module, i just saw the single <<eacute::utf8>> = "é" example. but nothing more.
|> Pattern matching guide. nothing here about binaries
|> Binaries, strings, and charlists guide. There is pattern matching here but nothing signposting the user to where there is more information
|> Scrolled down got to the mix-otp section and figured i would have seen something at this point
|> googled it, Binaries, strings, and charlists guide is the top result. there are some articles but none link back to the docs
|> asked on discord. got linked to the <<>>/1 special form docs.

Then when i was going to make this pull request, doing a search in github i found the patterns and guards reference which is at the very bottom of the guides and this does link to the special forms docs

[whatyouhide]

I think you can link to the guide as well:

Suggested change

	See the documentation for [`<<>>`](`<<>>/1`) or the Patterns and Guards
	guide for more information on binary pattern matching
	See the documentation for [`<<>>`](`<<>>/1`) and the [*Patterns and Guards*
	guide](patterns-and-guards.md) for more information on binary pattern matching.

Can you try out make docs_elixir locally with this and see if the link is to the right page?

[sabiwara]

.md perhaps?
Another instance from Kernel doc:

[Patterns and guards](patterns-and-guards.md)

[whatyouhide]

Ah sure yep

[DeemoONeill]

done. built the docs and it links to the correct pages

[DeemoONeill]

Thanks for the feedback I'll sort that out when I'm home.

I might change the order too, "see patterns and guards (link) and <<>> for more information on binary pattern matching. "

Go from less specific to more specific.

[josevalim]

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