Updates descriptive text that refer to Zulip channels in the
`docs/contributing` files to use channel instead of stream.
Part of the stream to channel rename project.
- Updates instances of "private message", "PM", and "private_message",
excluding historical references in `overview/changelog.md`.
- Also excludes `/docs/translating` since we would need new
translations for "direct messages" and "DMs".
* 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.
Lauryn Menard
David Rosa
Alya Abbott