Add an 'Explanation' Overview document

[sei-renae]

The target audience for the Explanation is a non-expert, and per diataxis.fr, should be able to casually read this and get an overview of what is SSVC.

This PR: Adds the Explanation document Modifies the main index page cards to Add the Explanation and Delete Learning SSVC Modifies the menu bar via mkdocs.yml per index page cards Moves Learning SSVC to Reference

[sei-renae]

Also at https://github.com/sei-renae/SSVC/discussions/1

[ahouseholder]

Still need to fix these:

URL        `topics/decision_trees/'
Name       `Decision Trees'
Parent URL file:///home/runner/work/SSVC/SSVC/site/ssvc_overview/index.html, line 3943, col 314
Real URL   file:///home/runner/work/SSVC/SSVC/site/ssvc_overview/topics/decision_trees/
Check time 0.000 seconds
Result     Error: URLError: <urlopen error [Errno 2] No such file or directory: '/home/runner/work/SSVC/SSVC/site/ssvc_overview/topics/decision_trees/'>

URL        `howto/coordination_triage_decision/#coordinator-triage-units-of-work'
Name       `CERT/CC Coordinated Vulnerability Disclosure (CVD) project'
Parent URL file:///home/runner/work/SSVC/SSVC/site/ssvc_overview/index.html, line 3954, col 48
Real URL   file:///home/runner/work/SSVC/SSVC/site/ssvc_overview/howto/coordination_triage_decision/
Check time 0.000 seconds
Result     Error: URLError: <urlopen error [Errno 2] No such file or directory: '/home/runner/work/SSVC/SSVC/site/ssvc_overview/howto/coordination_triage_decision/'>
10 threads active,    [37](https://github.com/CERTCC/SSVC/actions/runs/15495231228/job/43630238573?pr=716#step:7:38) links queued,  795 links in 842 URLs checked, runtime 11 seconds

URL        `reference/index'
Name       `Reference material'
Parent URL file:///home/runner/work/SSVC/SSVC/site/ssvc_overview/index.html, line 4220, col 4
Real URL   file:///home/runner/work/SSVC/SSVC/site/ssvc_overview/reference/index
Check time 0.000 seconds
Result     Error: URLError: <urlopen error [Errno 2] No such file or directory: '/home/runner/work/SSVC/SSVC/site/ssvc_overview/reference/index'>

URL        `howto/tree_customization'
Name       `Guidance for customizing a Decision Tree'
Parent URL file:///home/runner/work/SSVC/SSVC/site/ssvc_overview/index.html, line [42](https://github.com/CERTCC/SSVC/actions/runs/15495231228/job/43630238573?pr=716#step:7:43)21, col 4
Real URL   file:///home/runner/work/SSVC/SSVC/site/ssvc_overview/howto/tree_customization
Check time 0.000 seconds
Result     Error: URLError: <urlopen error [Errno 2] No such file or directory: '/home/runner/work/SSVC/SSVC/site/ssvc_overview/howto/tree_customization'>