Split regexes page and add regexes best practices page

[chsanch]

The problem

As is indicated in Issue #2410:
Here's the list of the heaviest pages in the language directory:

-rw-rw-r-- 1 jmerelo jmerelo 93890 oct  8 07:30 operators.pod6
-rw-rw-r-- 1 jmerelo jmerelo 85838 oct 23 19:30 regexes.pod6
-rw-rw-r-- 1 jmerelo jmerelo 66345 sep 15 12:23 traps.pod6
-rw-rw-r-- 1 jmerelo jmerelo 65898 oct 11 08:14 5to6-perlfunc.pod6
-rw-rw-r-- 1 jmerelo jmerelo 57482 oct  7 12:07 5to6-nutshell.pod6
-rw-rw-r-- 1 jmerelo jmerelo 46332 oct 23 19:30 variables.pod6
-rw-rw-r-- 1 jmerelo jmerelo 40456 oct 23 19:30 subscripts.pod6
-rw-rw-r-- 1 jmerelo jmerelo 38744 oct 23 19:30 glossary.pod6
-rw-rw-r-- 1 jmerelo jmerelo 38604 oct 13 11:40 nativecall.pod6

At 80K, regexes is the second biggest. It also features a mixture of tutorial, reference and "best practices"

Solution provided

This PR fixes #2410. Splits the Regexes best practices section into a new Regexes: Best practices and gotchas page. The sections on the Regexes page has changed, referencing the new page at the end.

[JJ]

Looks OK, it's just a new name because text has been translated verbatim, far as I can tell.