Better describe the RFC process

[anngvu]

In our docs, we currently have a terse statement:

The clinical data model development has involved a Request for Comments (RFC) process. Continued development, if necessary, will involve additional RFCs.

From the RFC retrospective, one suggestion is that we need to be clear what this process looks like with a section describing it. There has been discussion about a more "standardized" process for Sage, so we might see if a new and improved process will come out soon that can be adopted here. And flowcharts would be nice.

If not, we'll summarize what has been done so far with some new guidelines based on learnings.

@elv-sb @brynnz22:
Please feel free to help standardize this part of the docs based on what you're thinking about for MC2.

@cconrad8:
Optional thoughts