[DOCS] Add versioning support

[chrisj at rtems.org]

Hi,

These patches add versioning support that can be used in code blocks.

It will allow us to move to RTEMS 7 without needing wide ranging version
number changes. The documentation does have some things like the version
of a compiler or list of files that do change but we should look to refine
what we have in the documentation so we do not need endless updates.

I have done some of the changes and we have a lot more todo so please send
in patches.

Chris
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