Write more developer tutorials

[brainwane]

Expand developer documentation with more tutorials explaining how to do various types of projects.

Ideas:

• add a new translation
• cut a new release
• add a new emoji
• test Zulip with a new browser
• work with a new file format (for uploading and/or thumbnailing in previews)

[timabbott]

I think probably the tutorials I'd prioritize most (because they're things folks often want to do):

• Add a new backend route (should document our rest_dispatch + has_request_variables model)
• Make a UI change / Add a new section of HTML (via Django templates or handlebars) to the site
• Display an error message in some Zulip UI you've written (we have a standard way to do this with report_error, but it's not documented)

And I think these would be pretty useful as well:

• Add a new setting
• Add a new management command (could be mostly a pointer to Django)
• Add a new module to the Zulip frontend
• Contribute translations
• Modify the Zulip markdown syntax (would be the how-to complement to docs/markdown.md; might be a bit tricky to write just because it's somewhat complex)

[timabbott]

We now have implemented several of the ideas I suggested somewhere in our documentation. Things we should probably add include:

• Add a new module to the Zulip frontend
• Modify the Zulip markdown syntax (would be the how-to complement to docs/markdown.md; might be a bit tricky to write just because it's somewhat complex)
• Make a UI change / Add a new section of HTML (via Django templates or handlebars) to the site
• Display an error message in some Zulip UI you've written (we have a standard way to do this with report_error, but it's not documented)

[timabbott]

I think the things we had in mind when we opened this issue are solved with the "write a new view" thing. It's now just a random list of unrelated documentation enhancements we could make. @brainwane what do you think about a strategy of taking the list in my previous comment and opening an issue for each of those, and then closing this?

[timabbott]

The markdown piece is already in #1857; I opened #2232 for the error message one; I think the others we already have now. #1649 is for the template system; and I'm not sure we really need docs for adding a new module to the frontend (it kinda exists in the Node testing docs).