Include regex syntax in man page

[tournemire]

Ripgrep's manpage does not document its default regex syntax, instead redirecting to the regex crate's documentation. This makes offline use with non-trivial regexes impractical, as end-users do not necessarily have a local copy of the crate's doc.

[BurntSushi]

I think this has been requested before, but duplicating the regex syntax doc is quarrelsome for a couple reasons:

1. It is extremely long. It would be too long to include it in man rg IMO. Which means we'd probably need to ship a man rg-regex-syntax or something.
2. This would create two separate copies of the syntax documentation, which in turn means it'd be easy for them to get out-of-sync with one another.
3. The regex syntax doc is "plain" HTML/CSS/some-javascript. It is not hard to download it if you need an offline copy.