Add subtitle field and allow interactive specification of headers.

[matthewwardrop]

This patch-set improves the functionality around headers and their presentation.

In particular:

• Adds support for a subtitle field.
• Improves the rendering of headers (and removes the incorrect treatment of document title as a header; freeing up h1 for proper use).
• Improves readability by slightly increase font size and reducing maximum page width.
• Allows for interactive specification of missing but required headers (will be accessible via the new kp tooling).
• Slightly improves the styling associated with headers, which formerly could be smaller than the body text.
• Fixes requirement for updated_at header to be present even though it is later overwritten.

Auto-reviewers: @NiharikaRay @matthewwardrop @earthmancash @danfrankj

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✅ Build knowledge-repo 1.0.551 completed (commit 66c4b9bc3a by @matthewwardrop)

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✅ Build knowledge-repo 1.0.553 completed (commit 1364c54b65 by @matthewwardrop)

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✅ Build knowledge-repo 1.0.558 completed (commit 7e76c0aa1d by @matthewwardrop)

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✅ Build knowledge-repo 1.0.560 completed (commit 0c72103390 by @matthewwardrop)

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✅ Build knowledge-repo 1.0.561 completed (commit 19725e4d6a by @matthewwardrop)

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✅ Build knowledge-repo 1.0.564 completed (commit 84b3f1742d by @matthewwardrop)

[matthewwardrop]

@danfrankj I've checked this over pretty rigorously locally. Merging in now. Let me know if you see anything concerning retrospectively.