Specify Zulip coding conventions/styles/approaches ZT follows

[neiljp]

Description of documentation

Documents such as https://zulip.readthedocs.io/en/latest/contributing/code-style.html are very good for Zulip as a whole, but inevitably include references to tools/languages that we do not use.

There are also cases where it is useful to call out elements that are specific to ZT, or emphasize particular aspects that come up repeatedly.

This may need discussion to refine, but a draft could pick out some definite points regarding at least:

• naming (general, ~private object methods, bool-returning, abbreviations, ...)
• docstrings (files, functions, classes, ...; which classes/functions benefit most/least)
• structuring functions (including *, and / in future)
• typing (possible link to Zulip doc for that - https://zulip.readthedocs.io/en/latest/testing/mypy.html)

As per the linked document, this is for helping clarify style that is not easy to enforce in some other way, such as via linters.

Most recently noted at:
https://chat.zulip.org/#narrow/stream/206-zulip-terminal/topic/Developer.20Docs.20.23T27/near/1785160