Add behavioral subfeature data guideline

[ddbeck]

Summary

This PR provides a general guideline for creating behavioral subfeatures.

Related issues

This continues a discussion started in #26605 (comment), while #26780 covers the partial implementation part of this.

This was partly inspired by #17352, but is narrower in scope.

[github-actions]

Tip: Review these changes grouped by change (recommended for most PRs), or grouped by feature (for large PRs).

[caugner]

LGTM so far.

Like #26780 already includes, some more examples might help comprehension.

[ddbeck]

@caugner I already mentioned 3 examples (css.properties.gap.flex_context and css.properties.gap.grid_context, and html.global_attributes.inert.ignores_find_in_page) and mention more specific guidelines (e.g., secure_context_required). Do you have something in mind here?

[caugner]

Here's what I had in mind:

Suggested change

	- A contextual support condition this is not readily feature detectable, such as a CSS layout context or the effect of [an IDL extended attribute](https://dontcallmedom.github.io/webidlpedia/extended-attributes.html).
	For example, if support for a CSS property is conditioned on the layout context where that property is used, create a subfeature of the property to represent the behavior (as demonstrated by `css.properties.gap.flex_context` and `css.properties.gap.grid_context`).
	- When support depends on a contextual condition that cannot be easily or accurately feature-detected.
	Examples:
	1. CSS layout context: The CSS property `gap` depends on the layout context, so there are behavioral subfeatures `flex_context` and `grid_context` to distinguish support (see: [css/properties/gap.json](https://github.com/mdn/browser-compat-data/blob/main/css/properties/gap.json)).
	2. Effects of [an IDL extended attribute](https://dontcallmedom.github.io/webidlpedia/extended-attributes.html) <!-- Add example -->

Same for the behavioral evolution.