Auto merge of #10578 - epage:add-docs, r=ehuss

Auto merge of #10578 - epage:add-docs, r=ehuss

Document cargo-add

### What does this PR try to resolve?

cargo-add's PR was minimal to ensure we had settled on the CLI before
adding references to the CLI and have cascading churn.  This the command
documentation and adds references to that command.

### How should we test and review this PR?

I'm unsure what testing is not automated for the documentation (like link checking).

### Additional information

To keep the PRs focused, I am submitting completions separate from documentation updates so one does not get blocked on the other and its easier to see all relevant parts and sign off on them.