chore(kno-11406): clarify partials usage

[cellomatt]

Description

This PR:

• Updates usage patterns for partials in light of recent improvements.
• Adds from_json helper to Liquid helper reference
• Adds JSON input type to partials schema reference, with example
• Adds clarity on <style> tags for HTML partials
• Moves a section about editing HTML partials (with preview pane) to the new HTML partial page
• Adds FAQ on default schema values in conditions to the partial schema reference, to help with "gotcha" of things like truthy empty strings in a Liquid condition

[linear]

KNO-11406 [Docs] Update Partials overview and FAQs to reflect latest improvements

[vercel]

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Project	Deployment	Actions	Updated (UTC)
docs	Ready	Preview, Comment	Feb 18, 2026 5:28pm

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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Lots of new changes, should be reviewed again before merging.

[samseely]

Great updates!

[emisilvacab]

Everything looks good! My only comment is that the json input type hasn't been released yet (see pr #7364). We should also make sure to add documentation for that under the message type variants.