add greendot algo for opsz axis

[m4rc1e]

Following PR replaces googlefonts/gftools#689.

[vv-monsalve]

Following the most recent Axis Registry meeting, it was decided that it would be acceptable to include the opsz extremes values in the STAT table, even if they differ from the fallbacks listed in the axis .textproto file.
We took Playfair (which includes 1200 in the STAT table) as a reference case.

[RosaWagner]

would be nice to come back to this at some point

[davelab6]

@m4rc1e please could you get this sorted

[kenmcd]

This is still not accessible:
David Berlow's Green Dot algorithm
https://docs.google.com/document/d/15652Yabs0prnocpjG1TxG6zrfFSIYqOwlSIMkdbgqfg/edit?resourcekey=0-DXNZQLV2TbSqyn9HCLfhFA

I assume that doc explains this (which would be nice).

[davelab6]

@kenmcd and all, I finally added the doc to https://googlefonts.github.io/gf-guide/green-dot.html

@m4rc1e please integrate that and finalize/merge this PR :)

[m4rc1e]

Sure thing.

[kenmcd]

Thanks.
Please add info on why it is called the "Green Dot" algorithm.
I found nothing relevant with web searches.

[m4rc1e]

Sorry for the delay on this. I will take a look tomorrow.