Adding sponsors to the docs.

[lovelydinosaur]

Quoting from #2680...

I'm making some changes to the Starlette documentation to make it more modern and appealing.

I intend to do the same with Uvicorn, and I intend to add a sponsors tab.

Yep, adding sponsors to the Starlette docs would be fantastic.

I'd suggest a unified approach across the encode docs?
Perhaps let's take an agile approach and deal with this incrementally.

We could start with a unified approach for httpx, starlette, and uvicorn...

• https://www.python-httpx.org/ (Docs have a sponsor tab)
• https://www.uvicorn.org/ (Docs have a sponsor tab)
• https://www.starlette.io/

Perhaps we could start with...

• Use the same layout for the Starlette docs as Uvicorn and HTTPX.
• Add "Fund Encode" links pointing to the sponsorship page?