CONTRIBUTING.md: out of date information about contribution process

[myitcv]

What version of CUE are you using (cue version)?

$ cue version
bd9ae5371cc23c54cb3cb90c0bd5a1fa4d82bc92

Does this issue reproduce with the latest release?

n/a

What did you do?

Linked to the contribution guide when creating a PR:

What did you expect to see?

The contents of the actual contribution guide, content that is currently sitting within https://github.com/cue-lang/cue/blob/master/doc/contribute.md.

What did you see instead?

Out of date information regarding the use of Gerrit, with an onward link to the actual guide at https://github.com/cue-lang/cue/blob/master/doc/contribute.md.

The problem from my perspective:

1. If there are any changes to the contribution guide in https://github.com/cue-lang/cue/blob/master/doc/contribute.md, these are not flagged to people when creating PRs as "the contribution guide has changed since you last submitted a PR".
2. People have to needlessly click through to the actual contribution guide
3. The content in https://github.com/cue-lang/cue/blob/master/CONTRIBUTING.md and https://cuelang.org/docs/contribution_guidelines/ isn't actually that useful by itself; the reader invariably has to click through to the actual contribution guide for more information.

Suggestion

• Move the content in https://github.com/cue-lang/cue/blob/master/doc/contribute.md to https://github.com/cue-lang/cue/blob/master/CONTRIBUTING.md, and deprecate the former by including a link to the latter.
• Update the text and links on https://cuelang.org/docs/contribution_guidelines/ to point directly to https://github.com/cue-lang/cue/blob/master/CONTRIBUTING.md