Documentation changes required for release

[Chris Johns]

Hello,

The following was add to the documentation build support for the RTEMS 6 
release to avoid the need for us to perform mindless updates of the 
documentation on each release cycle:

https://git.rtems.org/rtems-docs/tree/README.txt#n604

The text is:

10. Use the following to embed the version number in any part of the
     documentation source:

      1. @rtems-version@

         The complete version string of the documentation.

      2. @rtems-ver-major@

         The version major number.

      2. @rtems-ver-minor@

         The version minor number.

      2. @rtems-ver-revision@

         The version revision number.

     The replacement happens during the source read phase of the build
     and is not context specific. The substitution will happen in code
     blocks and other normally quoted area.

     It is a requirement these be used then embedded commands or
     related text in the documentation to let the documentation track
     the release. For example `microblaze-rtems6-gdb` should be written
     as `microblaze-rtems at rtems-ver-major@-gdb`.

Note, the support for these version number may require some adjustments 
of what is provided in the documentation to aid the text being moved 
across releases without change. There is no hard and fast set of rule so 
please use commonsense. Sometimes less is better but is it a piece by 
piece call.

I would appreciate this being followed rather than it being the 
responsibility of the people perform the release to sort out. It only 
delays the releases.

Thanks
Chris