docs: cannabilize sphinx documentation into main README

[sighphyre]

This collapses all the docs in the SDK into the README. The Sphinx documentation was really difficult to discover if you didn't know where to look and users shouldn't have to fish around on the Internet to get critical info on the SDK (and honestly I'm tired of dealing with Slack questions that could just be written down)

Changes

• All the details in the Sphinx docs are now in the main README
• Broke out DEVELOPMENT.md — just a copy-paste for now. If it needs real love, we can fix in another PR
• Added missing docs for every SDK constructor param, lots of important stuff here that was more or less invisible via the README
• Added bootstrap examples (dict, file, URL)
• Renamed “events” to "impression data" since that's currently their only use, added some proper examples here
• Updated custom strategies to the real, modern API. Docs weren't up to date here
• Dropped the docs for legacy fallback values and custom strategy APIs. Those technically still work but they've been deprecated for a while and I want people to stop using them going forward. Please bro. Please. Let them die
• Added explicit notes on what types work in Context. Users were previously guessing/inferring it from the type and getting it wrong

TODO

• Sphinx docs can probably be nuked now. I'm open to being told no here. This and Rust are the only SDKs that have rich documentation that's separate from the main README. Rust is less of an issue since the crate docs are the first step for Rust nerds but for Python I think it makes sense that the main Unleash docs are a one stop shop
• DEVELOPMENT.md probably wants a second pass but I want this to settle first

[coveralls]

coverage: 95.475%. remained the same
when pulling cf665fb on docs/cannabilize-sphinx
into 35ae106 on main.

[sighphyre]

The intro here is shamelessly stolen from the Node SDK. I don't know if we want this or not but it feels like a nice way to ease into the docs

[sighphyre]

There are a TON of ways of installing packages in Python. I know in node we list yarn and npm. For Python I don't think it's worth detailing every single one, since most Python programmers can extrapolate from pip out to their choice of package manager (it's pip all the way down anyway)

[sighphyre]

There's no way to force a sync in the Python SDK

[melindafekete]

Thank you @sighphyre this looks great!