Add an embed role

[tylere]

Adds an embed role to include content inline.
Adds documentation.

Fixes #2646.

This is my first time trying to add a role, following the Adding a New MyST Feature docs. Claude Code helped too. Suggestions appreciated. I realize this may not be the best way to implement the feature, especially given the work on #1961.

[changeset-bot]

🦋 Changeset detected

Latest commit: 3c5de4b

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 12 packages

Name	Type
myst-common	Minor
myst-roles	Minor
myst-cli	Minor
myst-config	Minor
myst-frontmatter	Minor
myst-spec-ext	Minor
myst-parser	Minor
myst-directives	Minor
myst-to-html	Minor
mystmd	Minor
myst-migrate	Minor
myst-transforms	Patch

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[tylere]

This issue has a failing check because it is not labeled. It should be labeled as "enhancement", but I don't have sufficient permissions to add the label.

[rowanc1]

This is awesome - very into this feature having a role that is equivalent to the directive makes a ton of sense. Haven't had a chance to look at it specifically yet though!