Migrate description of the standard from pmix.org

[SteVwonder]

The pmix standards page: https://pmix.org/pmix-standard/

Probably don't want to include the re-hosted RFC pages, either just link to the github versions, or not include them since we are sun-setting the RFCs.

[naughtont3]

I went ahead and converted everything. We can decide if this stays in pmix or moves to openpmix pages, or just gets dropped entirely.

[abouteiller]

All information from that page https://pmix.org/pmix-standard has been imported.

I'd like to remove the origin page (or redirect to pmix.org/standard) but this pmix-standard page is not generated from Jekyll?