readme docs: confusing help for `aio config`

## From a user:

Proxy support section on this page https://github.com/adobe/aio-cli?tab=readme-ov-file#proxy-support says set environment variable (refer image) and same page has a section dedicated to setting up environment variable (refer image). That is causing confusion.

I was trying to setup environment variable as following which are both wrong.
aio config -e HTTP_PROXY=https://user:password@example.com/
aio config -e HTTP_PROXY https://user:password@example.com/


## Explanation

`aio config` is actually an alias for `aio config list`, but it also lists `get`, `set`, `edit` etc in its help. Users are confused here since `aio config` also lists the whole help for `aio config` (and its subcommands), as well as it being the alias for `aio config list` and showing the flags for that.

Start with the Proxy support and environment-variable sections in the README, then inspect the `aio config` help entry point and its `list` alias. Clarify which command users should run and ensure the README and help output do not present conflicting guidance.

Written by the indexing model from the issue text.