Update C++ style guide
The canonical source for the C++ style guide has been migrated from HTML
to Markdown, so there are a significant number of non-content changes
in this commit as well.
- Clarify code examples are for illustration only and not meant to be
copied verbatim
- Document when it makes sense to use classes to group static members
rather than having free functions in a namespace
- Add `kConstant` examples of constant template parameters and
`static constexpr` class data members
- Note that empty loop bodies (`{}`) are still subject to the general
line-breaking rules
- Other minor wording and formatting updates
