Adding maintainer responsibilities, off-boarding, and emeritus governance docs

[aburdenthehand]

1. Adding maintainer responsibilities, since we mention this as part of our selection criteria.
2. Also adding off-boarding/retiring expectations for maintainers, something we've been talking about throughout the year but never put into our docs. It is also a part of CNCF graduation criteria, so let's put it into our docs.
3. Also also adding an opening para that links to our membership policy, to make that more obvious for anyone looking for that info in our governance doc. This is mirrored by adding project maintainers to our contribution ladder in the membership policy, linking back to governance, and to our maintainers file.

Fixes #311

As per our governance, this requires a 2/3 vote of maintainers to update.
/cc @fabiand @davidvossel @rthallisey @rmohr @vasiliy-ul @vladikr @stu-gott

Release note:

Adding responsibilities and offboarding expectations for maintainers

[vladikr]

Thanks @aburdenthehand
We discussed this offline as well.
/lgtm

[kubevirt-bot]

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[iholder101]

Thanks @aburdenthehand!
Looks great to me. Left some nits.

/lgtm

[stu-gott]

👍 from me

[rthallisey]

+1

[fabiand]

Thanks for the work on reshaping the governance model.
All of this is going into the right direction!

It's a living document, follow up changes can be suggested and will be reviewed.

/approve

[kubevirt-bot]

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[vladikr]

/lgtm

[kubevirt-bot]

/remove-label needs-approver-review