Auto merge of #62549 - ehuss:update-cargo-vendor, r=alexcrichton

Update cargo-vendor usage

This contains a variety of updates to clean up the usage of cargo-vendor.

- Remove the install step for the old cargo-vendor now that it is built-in to cargo and available in the stage0 install.
- Update installation instructions, dealing with vendoring. The current instructions of running `sudo ./x.py install` is broken, it will almost always fail (since the vendor directory doesn't exist). Since the steps for properly handling this are numerous, I'm recommending removing the suggestion to use `sudo` altogether.
- If the sudo-forced-vendoring detects that the vendor directory is not available, abort with instructions on how to fix.
- Now that cargo-vendor is built-in, automatically run it if it looks like it is needed.
- Update instructions on how to install cargo.
- Remove the unused markdown link references in README/CONTRIBUTING. This reverts most of #44935. These references don't do anything if they are unused.

Closes #49269
cc #61142 #48771 #40108

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