Updates descriptive text that refer to Zulip channels in the
`docs/outreach` files to use channel instead of stream.
Part of the stream to channel rename project.
* Make an "Outreach programs" documentation directory.
* Revamp doc on having an amazing outreach program experience.
* Extract an outreach programs overview page from GSoC guide.
* Add a guide on making PRs easy to review, extracted from
"How to have an amazing summer with Zulip".
* Create a guide for mentors, extracted from "How to have an amazing
summer with Zulip".
* Add a guide on how to ask great questions extracted from GSoC guide.
* Extract general page on applying to outreach programs from GSoC guide.
* Simplify GSoC guide page to just describe project ideas.
* Many local edits to the reorganized content.
