docs: update the CLI usage documentation

[rdimitrov]

The documentation about the usage of both repository and client side CLIs should be refreshed so it matches the current features which go-tuf provides.

The things that need to be fixed are the following (nevertheless it won't hurt if we have a look at the rest too):

• tuf-client init does not work with root keys anymore, but instead uses the root.json file as per the spec (Thanks to Alex Khanin for raising that in #tuf (CNCF Slack channel)).
• In the same context, tuf root-keys command is obsolete too.
• tuf regenerate is not implemented yet, so maybe we can remove it if it's something we don't need or plan for the time being?

Note:
Removing the root-keys command might also be followed by removing the init implementation we kept as being backwards compatible. For reference - #210 and #208

[keerthi-nallaginela]

Hi, Can I take this up?

[rdimitrov]

@keerthi-nallaginela - Of course! Let us know if there is anything we can help you with 👍

[rdimitrov]

Closing since the code base changed and this is no longer valid.

Thanks for raising this 👍