Add `forbid-docs-links` ESLint rule

Created by: umpox

Slack context: https://sourcegraph.slack.com/archives/C0C324C91/p1641855680027300

We should be using /help in place of docs.sourcegraph.com for links in-product. /help will automatically version docs in the URL to serve a cached version-correct docsite

Note: Needs https://github.com/sourcegraph/sourcegraph/pull/32211 to be merged into this branch first

Test plan

eslint --fix - Replaced all incorrect usage. See this PR for result: https://github.com/sourcegraph/sourcegraph/pull/32211