Update links to the handbook and developer docs

Created by: valerybugakov

Problem statement

Some links to the documentation trigger redirect and some links are broken.

Success criteria

1. Most handbook links can be updated from https://about.sourcegraph.com/handbook/* to https://handbook.sourcegraph.com/*.
2. Some handbook links should be updated from https://about.sourcegraph.com/handbook/* to https://docs.sourcegraph.com/* e.g., Go style guide should point to https://docs.sourcegraph.com/dev/background-information/languages/go

Things to learn/practice during the implementation

Learn more about Sourcegraph documentation.

Estimated amount of work

T-shirt size estimate: S.