Document ossec-regex -p for PCRE2 patterns

[atomicturtle]

Summary

• Document ossec-regex -p / --pcre2, which tests a pattern as PCRE2 with the same flags as a <pcre2> rule.
• Note that the default mode is still legacy OSSEC regex and match syntax, so ., *, and { are rewritten before matching.
• Point the PCRE2 syntax page at ossec-regex -p.

Code: ossec/ossec-hids#2320

Test plan

• Review docs/programs/ossec-regex.rst against ossec-regex -h and a -p run
• Confirm the :ref: links to regex and ossec-regex-convert resolve