docs(changelog): add the missing 1.5.0 entries and fix its markup (#1103) (#1767)

Entries:
- add 69 entries for merged changes the 1.5.0 file did not cite, most of
  them from #1196 on: the round-trip fixes, reassembly, the engines,
  registration, the config module, and the corekit moves
- add a pcapkit.interface section for its new registry helpers
- cover breaking changes the file had missed: AppType.proto,
  TransportProtocol's base, TLSKeyLabel's move, Method.get case, the
  sentinels, the all extra, IPv6_Ext and register_mh_extension

Accuracy:
- the ftp/http case-folding entry notes that Method.get matches case
- the construction-keyword entry says a keyword passed to from_data raises

Preamble:
- re-measure the citation figures and list the 1.5.0b4 to b12 prereleases
- drop the "as of this revision" snapshot wording

Markup:
- un-nest seven inline literals from bold, and close an unterminated literal
- regenerate CHANGELOG.md with util/changelog_md.py

Conventions:
- re-pin process.rst's changelog figures to 10 sections and 269 entries

Refs #1103

Author: JarryShaw