[bugfix] Relax Mention parsing, allowing either href or name

[tsmethurst]

Description

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If this is a documentation change, please briefly describe what you've changed and why.

This pull request relaxes the format required for incoming Mention tags, by making the name property no longer a strict requirement. Instead, GtS will now try to use href first (either URI or URL, both should work), and fall back to name if that's not set. If neither are set, the mention will be discarded.

Also adds some documentation for this in our "Federating with GoToSocial" document :)

closes #2144

Checklist

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• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.