[docs] add C++17 dependency, fix markup issues

[cz4rs]

std::optional is documented as being added in C++17, while Magistrate documentation says it's a C++14 library.

Fortunately, it doesn't look like std::optional is actually used anywhere in here, so this inclusion should just be dropped.

Also, maybe that line of the documentation is out of date, in which case, please open an issue. Additionally, I see two more issues of documentation markup not being rendered properly, which may represent additional issues(s):

• \checkpoint
• @m_class{m-note m-dim}

Originally posted by @PhilMiller in #345 (comment)

TODO:

• magistrate requires C++17, update the docs to reflect that.
• Take a look at the rendering issues mentioned by Phil and fix them or open a separate issue.

[cz4rs]

Make sure to sync m.css and Doxygen version with vt. See DARMA-tasking/vt#2367 and DARMA-tasking/vt#2368 for more details.