Standardize C++ Headings

[SethFalco]

What does this PR do?

Improve repo

For resources

Description

C++/CPP/CPlusPlus/C Plus Plus is written in several ways in the repository. This is mostly to accommodate the confliction with C as they both get the same ID after the Markdown is parsed and transformed into HTML, so the 2nd one gets assigned c-1 instead.

Even this has been done incorrectly for some files (free-programming-books-cs.md) as c-1 doesn't mean C++, it means the 2nd heading called C, which doesn't exist if there is a C++, but no C in the file.

I noticed a particular solution in the repository that stood out, however.

### <a id="cplusplus"></a> C++

- https://raw.githubusercontent.com/EbookFoundation/free-programming-books/d703553631a600511a82035a1d736a206d80661c/books/free-programming-books-id.md

Instead of playing with the wording, or trying to accommodate the automatic IDs, it could be nice if for a case like this we just explicitly assign an ID to C++ instead by using the HTML equivalent of ###.

Why is this valuable (or not)?

I think this would be a great way to standardize how we add C++ in the repository in a way that semantically differentiates from C both as a reader, and for machines.

Checklist:

• Read our contributing guidelines
• Search for duplicates.
• Include author(s) and platform where appropriate.
• Put lists in alphabetical order, correct spacing.
• Add needed indications (PDF, access notes, under construction)

Followup

• Check the output of Travis-CI for linter errors!