Add links to the CMIP6_CV github from Control Vocabulary (CMIP6)

[jypeter]

I think that it would be nice to have links to the appropriate github pages from CMIP6 Control vocabulary minimum requirements.

e.g. replace institutions_ids with

"institutions_ids [ raw ]"

[mauzey1]

I moved this issue from https://github.com/PCMDI/cmor since this should be the more appropriate place for it.

[jypeter]

OK, makes sense!

[durack1]

It might be useful to replace the existing "CMIP6 tables issues" with "Valid CMOR3 managed tables"

And include
https://github.com/PCMDI/mip-cmor-tables/
https://github.com/PCMDI/cmip6-cmor-tables/
https://github.com/PCMDI/input4MIPs-cmor-tables/
https://github.com/PCMDI/obs4MIPs-cmor-tables/
https://github.com/metoffice/arise-cmor-tables/

Then this issue can be closed out