Troubleshoot.sh Documentation updates for the URI field

[xavpaice]

Separated the documentation task from #694 and #695.

We would like to see the docs for Troubleshoot updated to include the usage of the newly added URI field in supportbundle type specs.

Coverage required:

• A new node under the "Support Bundle" heading (below https://troubleshoot.sh/docs/support-bundle/collecting/) which describes how to use the URI field to point Troubleshoot at an upstream URI for updates.
• Description in that page of how the URI field behaves if the retrieval is successful, and if it is not - i.e. replaces the entire spec in that yaml with the downloaded one, how it behaves if there are multiple specs on the CLI, and what happens with redactors.

This will require some experimentation to confirm.

Definition of done:

• [ ] Documentation updated to show the purpose, usage and behavior of the URI field in https://troubleshoot.sh/docs
• [ ] CLI switch usage from #695 is documented in the same place