Add adk flags and env variables to docs

[zeroasterisk]

Is your feature request related to a problem? Please describe.

Unable to find CLI flags like --trace_to_cloud documented anywhere.

While this might be hard to keep up to date, we could include a few important ones and a link to get more from the source:

https://github.com/google/adk-python/blob/577222509c0f7ff7bd1fa4317eb20fd871a37daa/src/google/adk/cli/cli_tools_click.py

[koverholt]

@hangfei, do you (or someone you can refer me to) have a better idea how we can make the CLI subcommands and flags more discoverable for users? Aside from changes in how we're using click in our CLI tool, anything that you would suggest in our docs that we can either autogenerate from API docs or click that we could use in the docs, rather than manually traversing the CLI?