doc: add page on client configuration file and vars

Context

Currently, the documentation mentions nowhere that the client can be configured using a configuration file. This MR aims filling this hole by introducing a new page about the client configuration, inspired from the existing one on the node configuration.

Incidentally, the new page also recovers the set of environment variables for the client, that were somewhat misplaced in the "Getting started" tutorial.

Manually testing the MR

In the doc artifact, check that:

• page user/setup-client.rst has been enhanced to introduce the new client configuration page
• this latter page is correct, complete enough, and readable
• the modification in the pages for other tools (the admin client, baker, accuser, rollup node, DAC node, and DAC client) are correct.

Checklist

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• Select as Assignee the next person who should take action on that MR