docs(cmp): Document ConfigManagementPlugin.spec.version more clearly

[peschmae]

Adds a description to the spec.version attribute in the CMP, to make it clearer that this is the version of the plugin, and is optional.

This confused me quite a bit in the begining, and there were already tickets about this, eg. #13034

Checklist:

• [x ] Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
• [x ] The title of the PR states what changed and the related issues number (used for the release note).
• [ x] The title of the PR conforms to the Toolchain Guide
• [ x] I've included "Closes [ISSUE #]" or "Fixes [ISSUE #]" in the description to automatically close the associated issue.
• I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them.
• Does this PR require documentation updates?
• [ x] I've updated documentation as required by this PR.
• Optional. My organization is added to USERS.md.
• [ x] I have signed off all my commits as required by DCO
• I have written unit and/or e2e tests for my change. PRs without these are unlikely to be merged.
• My build is green (troubleshooting builds).
• My new feature complies with the feature status guidelines.
• [ x] I have added a brief description of why this PR is necessary and/or what this PR solves.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a43b799) 49.55% compared to head (661e471) 49.43%.
Report is 21 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #15974      +/-   ##
==========================================
- Coverage   49.55%   49.43%   -0.12%     
==========================================
  Files         269      269              
  Lines       46641    46790     +149     
==========================================
+ Hits        23113    23131      +18     
- Misses      21256    21385     +129     
- Partials     2272     2274       +2     

see 9 files with indirect coverage changes

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[peschmae]

Rebased to master

[crenshaw-dev]

Maybe just a bit more explicit?