doc: formalize `auto` usage in C++ style guide
Adjust heading levels to align with the table of contents, and write down 2 more existing rules (avoiding non-const references and auto
under most circumstances).
We generally avoid using auto
if not necessary. This formalizes this rules by writing them down in the C++ style guide.
The other part of the PR has been split out into #23155.
Checklist
-
make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
documentation is changed or added -
commit message follows commit guidelines