doc: add cli documentation

Created by: LawnGnome

This roughs in some top level src-cli documentation, including:

• Expanded src login documentation.
• A description of the environment variables used by src-cli.
• The Windows specific documentation previously only found in the src-cli repo.
• Information on creating access tokens.
• A bare bones command reference.

I'm on the fence about the utility of the reference: it's auto-generated from src-cli via an experimental doc command added in https://github.com/sourcegraph/src-cli/pull/387, but we don't really have the structured data in src-cli right now to drive a good Markdown experience. Opinions welcome.

I've pushed this to https://docs.sourcegraph.com/@aharvey-login-docs/cli (alas, I forgot until after I'd opened this PR that docsite can't handle slashes in branch names, so I'll have to try to keep that up to date separate to this PR by double pushing).

This closes #14528 (closed) and closes #15882 (closed).