docs: Drop the regex-syntax page

Point people to the official PCRE documentation instead, which is going
to be more up to date. This saves us periodically having to copy in and
reformat the PCRE documentation.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Helps: #3037

glib/gregex.c

@@ -45,7 +45,8 @@
  * A `GRegex` is the "compiled" form of a regular expression pattern.
  *
  * `GRegex` implements regular expression pattern matching using syntax and
- * semantics similar to Perl regular expression.
+ * semantics similar to Perl regular expression. See the
+ * [PCRE documentation](man:pcrepattern(3)) for the syntax definition.
  *
  * Some functions accept a @start_position argument, setting it differs
  * from just passing over a shortened string and setting %G_REGEX_MATCH_NOTBOL