doc: allow <code> in header elements

[Trott]

First commit:

doc: allow <code> in header elements Allow use of <code> in header elements without styling side effects. We can add styling later if desired. The goal here is to allow code to be set off in markdown without needing to escape characters and do lint exceptions for terms. This is probably a win in terms of accessibility too although it would be moreso if we had some visual differentiation for <code> inside of headers. As mentioned above, that can always be added at a later time. 

Second commit:

doc,dns: use code markup/markdown in headers This will allow us to lint for use of `hostname` in prose without flagging `hostname` in code within headers. This also allows us to remove backslash escaping for `[` and `]` inside of header code, which makes the bare markdown more readable. 

Subsequent commits make the change for all the other subsystems.

@nodejs/documentation

@nodejs/website (note the CSS changes)

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• documentation is changed or added
• commit message follows commit guidelines

[Trott]

Probably easiest to review by looking at the first commit in isolation, and then the second commit in isolation. All the other commits follow suit from the second commit.

[marswong]

LGTM

[marswong]

Class: Options might be more harmonious than Class: Options

[Trott]

Had to make a small change to tools/eslint-rules/documented-errors.js so that it would pass with the ` marks added to errors.md.

[nschonni]

LGTM, I've seen this over on the MS Docs that they use this approach as well because it is also an indicator that these are chunks that shouldn't be translated

[BridgeAR]

RSLGTM