Document the current component discovery + SDC criteria + `Component` config entity, and describe in an ADR

Overview

Sibling of #3461490: Document the current JSON-based data model, and describe in an ADR for all things components — i.e. completely ignoring the data model, but instead focusing on how components (currently only SDCs) are discovered and made available in XB.

Just like #3461490 provided an overview to the data modeling person (site builder + back-end developer, if you will), this should provide an overview to the component tree creating person (site builder + front-end developer + content creator).

(Note that it's different from #3446083: Document supported component modeling approaches in that this one is about technical specifics, whereas that one is about the component modeling XB aims to support.)

This should tie together/provide an overview of not just the still-in-progress-but-will-happen-soon things (such as #3463999: Auto-create/update Component config entities for all discovered SDCs that meet XB's minimum criteria), but also the many future steps we need to take to leap beyond SDCs:

• (✅ == included in the docs)
• ✅ #3454519: [META] Support component types other than SDC, block, and code components → children: #3462244: Allow synthetic component definitions for BC sake, #3462241: [PP-1] Decorate the SDC plugin manager and allow components defined in code, #3462242: Remove tight 1:1 coupling between config entities and components: allow multiple Component config entities using the same SDC
• ✅ #3462705: [META] Missing features in SDC needed for XB
• ✅ #3446083: Document supported component modeling approaches

This ADR, just like that in #3461490, will be superseded. But it's important to capture the status quo, especially prior to DrupalCon Barcelona aka #3454094: Milestone 0.1.0: Experience Builder Demo.

Proposed resolution

User interface changes

None.