docs: replace release process document

[ericswanson-dfinity]

Description

The release process document that we have goes into some unnecessary detail, for example:

• which git command to execute in order to create a branch or check out an existing branch
• how to create a PR using the GitHub UI (click the link in the console to go to your new branch in GitHub, then click *Create Pull Request*)

It's also out of date, with references to nix.

Further, it's missing some important parts, like where to update the portal docs and to remember to update the sdk submodule there.

Also, we no longer use a "driver" and a "validator," because the release process no longer involves running a bunch of nix commands manually.

This PR replaces release.adoc with release.md. My intention is to describe the current process at a high level, with enough detail to be able to execute the steps, while assuming that the reader knows how to use git and the GitHub UI. As we streamling the release process, I think this document will prove easier to update with the improved workflows.

How Has This Been Tested?

I ran markdownlint on it

Checklist:

• The title of this PR complies with Conventional Commits.