Improve documentation for building Tor with Rust
We have documentation that has thus far been a work in progress at https://trac.torproject.org/projects/tor/wiki/RustInTor and https://trac.torproject.org/projects/tor/wiki/RustInTor/Hacking
However, we should create a doc/HACKING/RustInTor.md with basic information to get people started, as it isn't obvious to look at the above wiki pages. Also, there is still missing information, such as which dependencies are necessary if someone chooses to build using a specified local directory for Rust dependencies.
For example, we should have:
- Which dependencies are needed if someone wants to build using local Rust dependencies.
- Basic information about how to build and run tests
- Links to wiki pages that are still a work in progress
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