Add PROCESS.md documenting the new SDLC

[jdesrosiers]

Resolves: https://github.com/json-schema-org/json-schema-spec/issues/1334

This is the formal documentation of the process we've discussed in https://github.com/orgs/json-schema-org/discussions/234. That discussion was focused on getting agreement on the general direction, while this makes some concrete decisions. Please do challenge any of the concrete decisions I've made here, but keep in mind that this is just a starting point, so it might be more productive address issues after merge as separate PRs/Issues so we don't end up with multiple threads of discussion in this PR.

Several more concrete decisions still need to be made that should be addressed in the spec (for example: dialect and vocabulary URIs). Those decisions are out of scope for this document.

I made some minor changes from what we had discussed to how STAGE-2 is defined, so pay particular attention to that section when reviewing.

[gregsdennis]

@Relequestual have a read through the comments that I made and the responses given. Maybe that will answer some of yours.

[Relequestual]

@jdesrosiers Can this be closed in favour of https://github.com/json-schema-org/json-schema-spec/pull/1500 ?

[jdesrosiers]

Yes, it can