Doc: Use variable substitution for Mechanical version in doc build

We are using current mechanical versions in lot of places. Ideally it should be done when sphinx build happening. Currently there is no way to do variable sub inside code block ( as far as I know. I might be wrong).

Start by locating the Sphinx documentation build configuration and the docs that contain Mechanical version values. Check how substitutions are currently handled, then make the version available during the build, including where it appears in code blocks. Done means the documentation build succeeds and no longer requires repeated hard-coded version updates.

Written by the indexing model from the issue text.