Add doc for compresslevel of the GZipMiddleware and use compresslevel=5 in the example

[RaphaelMarinier]

compresslevel is defined there in starlette's GZipMiddleware:
https://github.com/encode/starlette/blob/4e453ce91940cc7c995e6c728e3fdf341c039056/starlette/middleware/gzip.py#L30

compresslevel=5 is typically a better CPU / compression level tradeoff than 9, e.g. see:
https://tukaani.org/lzma/benchmarks.html

[tiangolo]

📝 Docs preview for commit 1ba7288 at: https://12056547.fastapitiangolo.pages.dev

[soulee-dev]

Seems like it's duplicated PR #11350

[RaphaelMarinier]

Indeed, it's mostly a duplicate (I was not aware of it), except that the default I propose in the doc is probably a better tradeoff (typically 3x less CPU usage, for only one percent point larger outputs).