PCRE: Tidy up and clean-up some outdated things

[joshtrichards]

TL;DR:

• Updated content and links to match current version(s)
• Dropped material duplicated from elsewhere
• Minor language refactoring for clarity

Specifics:

• Clean-up "Perl Differences" docs
  • Removed the VERY outdated list of differences (it's from <PCRE1 4.0!). Replaced it with a link to the current official PCRE2 documentation on the topic.
  • Used a link since the old content was just a verbatim snapshot of the same docs (albeit an older version). It wasn't adding any value being copied directly here (and would quickly just get out of date).
  • Also, added a note about the relevance of the PCRE2 library version.
• Revise PCRE extension preface
  • Add actual introductory paragraph
  • Drop syntax details which are already thoroughly documented elsewhere (and already linked)
  • Clarify expression caching
  • Update size limitations link point at PCRE2 one
  • Drop stale final paragraph that didn't belong and is redundant
• Revise PCRE extension installation instructions
  • Remove the outdated configuration option; replace it with the supported option
  • Tidy up the opening paragraph(s)
  • Update the required PCRE2 external version to match the config implementation
  • Tidy up JIT paragraph
  • Tidy up "changes" paragraph; replace changelog link to point at the one for PCRE2 rather than the deprecated/unmaintained PCRE1 one
  • Add PHP 8.3.0-8.5.0 entries to bundled PCRE library history

Note: Depends on php/doc-base#256 for URL changes/additions. That should be merged first.