Add httpx-retries to third party packages docs

[will-ockmore]

Summary

httpx-retries is a third-party package which implements request retry.

There's been extensive discussion on this feature in https://github.com/encode/httpx/issues/108 - take a look at the end of the thread (specifically, the recent comment I left) for the current state of play.

I've added the package to the third-party packages docs; I've also sorted the entries alphabetically, as there's quite a few, to make it easier to browse (if this is an undesired change, let me know, I'll revert it!). I've run the docs locally, to verify everything looks good.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[lovelydinosaur]

I didn't +/- this initially because I was blocked by the awkwardness of some of the entries on this page.
Pointed towards a couple of obvious places.

They're unrelated to your addition, so we can either be neat and tidy and deal with different aspects in different PRs or else we can be pragmatic and deal with them up as part of this pull request. (Or we can be non-plussed)

[will-ockmore]

@tomchristie as I started the cleanup here by sorting the entries, I thought why not continue with the pragmatic approach.

I've included your comments, as well as some grammar improvements, and separated out libs that have HTTPX support from direct plugins.

An additional change we could make here if there's appetite is to remove duplication; there are two libraries for mocking, and two cache libraries.

[lovelydinosaur]

Lil' spruce up. Neato. 👍🏼
Not going to deal with de-dup just yet.