chore(docs): add httpx-retry to 3P packages

[mharrisb1]

Summary

Adds httpx-retry to third-party packages page in docs as suggested by issue comment in #108

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.

[zanieb]

Do you think it makes sense to add a package here before there's significant community adoption? (I don't feel strongly, but I was surprised to see a week-old package with one release)

[mharrisb1]

@zanieb up to you. Just following suggestion from issue thread. Feel free to close

[zanieb]

I don't mean to be discouraging — other people may feel like it's fine.

[mharrisb1]

surprised to see a week-old package with one release

This is valid. Maybe there should be a note on adoption threshold, minimum age, or something else to avoid this and other new projects that might become abandonware from being proposed or added.

Or maybe even something as simple as this:

# Third Party Packages
As HTTPX usage grows, there is an expanding community of developers building tools and libraries that integrate with 
- HTTPX, or depend on HTTPX. Here are some of them.
+ HTTPX, or depend on HTTPX. Here are some of the more popular ones.

I shilled the project in the comments so that should be fine for anyone landing there from a Google search that doesn't want to roll their own