Rust maintenance docs

[maxgmr]

Description

This PR is for a migration of my unofficial Rust Maintenance docs to the Ubuntu Project docs.

The docs consist of four components:

1. Updating Rust: the process of updating the Rust toolchain to a newer upstream version. Written by me.
2. Patching Rust: the process of making changes to a Rust toolchain which has already been uploaded to the Ubuntu archive. Written by me.
3. Backporting Rust: the process of backporting an existing Rust toolchain to an older Ubuntu release. Written by me.
4. Rust Version Strings: a detailed breakdown of the somewhat unique package version string used for Rust toolchain uploads and backports. Written by Petrichor Park (@petrak-at-canonical).

The existing Rust Maintenance docs have proven their utility by helping three new Rust team members become acquainted with Rust toolchain maintenance quickly.

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Checklist

• I have read and followed the Ubuntu Project contributing guide
• I have tested my changes, and they work as expected

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Additional notes

There are two things which may need to be addressed before this can be merged.

First, is the formality of the writing appropriate for the Ubuntu project docs? I use personal pronouns liberally throughout the docs in phrases like "You should consider XYZ", or "Next, we'll update ABC". I can make the tone more formal if necessary.

Second, I temporarily created a place for the docs in f0bd4c2 under "Maintainers" -> (NEW) "Niche Package Maintenance". Robert and I discussed possible category names for articles about specific complex packages, but we didn't settle on anything concrete (not to mention that the abbreviation for "Niche Package Maintenance", "NPM", could be confusing for Node devs). I'd like to discuss how the docs should be sorted/organized/displayed now that they're ready to be added.

Let me know if there's anything else that needs changing — I tried to match the existing style, but this is my first major contribution to the Ubuntu Project docs so I may have missed something. Thanks!

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