Version 2.7.1: LTO link fix

This patch release fixes linking against the precompiled shared library when it
is built with link-time optimization.

Fixed

• Fixed missing symbols in the precompiled library under LTO: Precompile.cpp
  now explicitly instantiates Option::ignore_case<App> and
  ignore_underscore<App>, and the library always compiles the full validator
  set unless CMake disables it. #1409

Documentation

• Added the changelog to the Doxygen site. #1405
• Titled the changelog entries and named the releases from them. #1406

Internal

• Moved the release checklist to a prepare-release skill. #1407