More documentation-related improvements

[Morwenn]

This issue is a direct follow-up to #157, a big picture issue about the future possible improvements of the tooling around the documentation.

Any change to the tooling in the direction of more automation and automatic checks or replacements should be thoroughly documented in the documentation page dedicated to tooling.

Future version placeholder

Currently we have to know the next version number in order to add comments such as New in version XXX or Changed in version YYY in the documentation. Following Catch2's lead, we could use a placeholder such as cpp-sort-version-next in the documentation file and have a small script replace those occurrences by the known version number. Such as script would be run before the release and would most likely help me not forgetting to change version numbers before a release.

MarkdownSnippets

MarkdownSnippets is a tool that allows to test the code embedded in Markdown files. Setting it up would ensure that all the code snippets in the documentation are actually correct.