Custom Select Picker Layouts v2

[LeaVerou]

This expands the content of this guide and addresses some of the issues I flagged here:

• Create guide and evals for the custom-select-picker-layouts use case #290 (comment)
• Create guide and evals for the custom-select-picker-layouts use case #290 (comment)

Changes related to the review issues:

• Add :open to code snippets and explicitly instruct agents to respect it (current guidance produced broken UI)
• Edited a11y guidance to instruct agents to fix spatial navigation instead of giving up on customizable select altogether. Ideally this needs to be reviewed by an a11y SME, but I'd be surprised if "reimplement the control with divs" is actually a better direction than fixing the keyboard navigation. I avoided providing an explicit snippet since it proved very hard to write a truly generic one, whereas agents knowing the specifics of their use case can write one that works for their case far more easily.

Other changes:

• Consolidate guidance for each step previously spread across three sections.
• Broke down the code example into separate smaller code snippets as needed to illustrate each step
• Deduplicate guidance previously mentioned in multiple places.
• Moved the guidance to use value attributes on options to the general best practices for customizable select.
• Add guidance for styling the checkmark, which was previously left unstyled and occupied space even on non-selected cards, edited the demo as well. Before and after below.

[LeaVerou]

@rviscomi ping!