💽 Add site options to each page

[rowanc1]

This enables per-page options for the site.

Picked up in the theme in jupyter-book/myst-theme#472

[changeset-bot]

🦋 Changeset detected

Latest commit: 62a5836

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

This PR includes changesets to release 6 packages

Name	Type
myst-frontmatter	Patch
myst-cli	Patch
myst-common	Patch
myst-config	Patch
myst-spec-ext	Patch
mystmd	Patch

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[fwkoch]

Validating options against the site template looks good and makes sense.

My only concern, though, is introducing site options, alongside the existing options field. In my mind, options covers all template-specific options including both site and static export templates... I'm worried about confusion around putting an option in the wrong options location.

[fwkoch]

How does this overlap with options, which already exists per-page?

[fwkoch]

Chatting some with @rowanc1 - we will keep the new site key, as a place to override site options at a page level. This is somewhat similar to how we treat other exports, where in the export definition, you can define other options and override fields. (There is some discussion of sites vs. exports here: #1103) Also, once we tackle removing the project key in myst.yml (#1308), page frontmatter with site alongside other keys will start looking very similar to myst.yml content with site alongside other keys.

As for options - these can remain, as a place to define any arbitrary key/value pair. The site/export templates should have access to these options, but they are less strictly validated than options defined directly on site.

Also note, I think this PR addresses: #1450