Mark implemented SLEPS

[lorentzenchr]

As of today, https://scikit-learn-enhancement-proposals.readthedocs.io/en/latest/index.html lists two SLEPs as accepted:

• SLEP009
• SLEP010

It is, however, not clear from the webpage if those SLEPs are implemented. I propose to link at least the scikit-learn version, maybe also relevant PRs, that implemented them.

[glemaitre]

In Python, there is a "Final" status to acknowledge that the PEP is implemented. We could do something similar by listing them separately and adding the scikit-learn version where the interface is stable.

[glemaitre]

I assume that it should be part of the SLEP000: https://github.com/scikit-learn/enhancement_proposals/pull/30/files

[glemaitre]

Actually, this something this is mentioned here and could be improved: https://github.com/scikit-learn/enhancement_proposals/pull/30/files#r758344193