c2nim, nim doc and doxygen

[FedericoCeratto]

Sometimes C sources processed by c2nim contain doxygen syntax e.g.:

## @mainpage My project
## @section intro_sec Introduction
## @param[in] flags Special options for this operation.

We could either have c2nim detect and strip them away or do it manually, but this is a bit wasteful because such comments are useful.

Alternative, nim doc could detect them and hide them from the output, as a first step.

In future, nim doc could use such comments to change HTML styling around sections, parameters and so on.

Partially related to #12353 #4517

[timotheecour]

related:

• https://github.com/nimterop/nimterop/issues/61
• https://github.com/nimterop/nimterop/commit/268b474332b0e0f4f1ca7cf296caf5039351f1e5