[python][doc] Review README.md for http-client-python before GA in March

[msyyc]

We shall make sure https://github.com/microsoft/typespec/blob/main/packages/http-client-python/README.md could work before GA in March, including all details.

[msyyc]

There are several places which may need update:
(1) package-dir is mainly used by tsp-client instead of tsp compile. Now that we recommend users to generate code by call tsp compile, we shall replace it with emitter-output-dir.
(2) when package-name is different with namespace, current emitter may don't work and we need fix it before GA. (Fix PR is in progress: #5649)
(3) packaging-files-dir/packaging-files-config is not tested yet, so not sure whether we shall expose them. Maybe we could expose them later after adding some test cases.