Move glossary definitions to yaml format

[parlough]

Converts the site-wide glossary to be hosted in a YAML file. Initially, only a few of the fields are used:

• term - for the header entry and default fragment
• id - (optional) to override the default fragment
• long_description - (but will fallback to short_description if missing)
• related_links - For linking to other docs

A few other fields have been added for experimentation but are not yet used or surfaced.

• short_description - A shorter description to be shown on hover
• category - A primary category for the term
• labels - Labels/topics that can be used for filtration
• alternate - Alternate and related terms that should lead to this entry. Could be because they are synonyms or that term doesn't yet have a specific entry of its own, but is still relevant enough.

Also new to the page:

• A horizontal line between entries to help visually separate them.
• A related links section after the long description.

The current style of this information will likely be completely replaced as more information is surfaced and filtering is added.

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This work is roughly based on the design proposed in https://flutter.dev/go/glossary-expansion and the Fuchsia glossary.

Resolves #6143

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Staged: https://dart-dev--pr6162-feat-6143-bema0uix.web.app/resources/glossary

[dart-github-bot]

Visit the preview URL for this PR (updated for commit 7830294):

https://dart-dev--pr6162-feat-6143-bema0uix.web.app

[sfshaza2]

lgtm

[MaryaBelanger]

Looking forward to this coming together!