Fixes a few typos / errors in commit a633890d8 that updated
these documentation articles on pull requests.
Also adjusts language around screenshots to match what we use
in the pull request template on GitHub for the zulip server and
web app repository.
Adds a 'Tips and best practices' section to the documentation on
writing reviewable pull requests.
Also, updates step 3 of the documentation on creating pull requests
to link to the new section and to not have an out-of-date screenshot
of the GitHub pull request template.
* 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