Augment C2178 error reference with `consteval`

[Rageking8]

• Merge and condense mutable remarks to make it clearer and give space to the new consteval remarks
• Clean up existing mutable example
• Add new remarks and example for consteval
  • The "Example: ..." heading is used instead of nested "Examples" heading as it adds the various examples to the "In this article" UI for quick navigation. It's especially useful for error/warning references with lots of example entries.
  • Here is the relevant quote in the draft spec: https://github.com/cplusplus/draft/blob/c3bd02a2e44c53c8fe7ba5b48a94bacc9186235d/source/declarations.tex#L943-L944
• Update ms.date metadata

I recommend to review one commit at a time.