Propose a new file structure (ADR) (#1678)

* Proposes a new file structure (ADR)

* Update contributor-docs/adrs/adr-XXX-file-structure.md

Co-authored-by: Cole Bemis <colebemis@github.com>

* More questions about the implementation process

* Doc updates; removal of open questions

* BreadCrumbs -> Breadcrumbs

* Remove note about separately exported subcomponents

* Update contributor-docs/adrs/adr-XXX-file-structure.md

Co-authored-by: Cole Bemis <colebemis@github.com>

Co-authored-by: Cole Bemis <colebemis@github.com>

Author: 2 people

contributor-docs/adrs/adr-XXX-file-structure.md

@@ -0,0 +1,41 @@
+# ADR XXX: File structure
+
+## Status
+
+Proposed
+
+## Context
+
+Components might be more grokable if they were structured consistently. This ADR proposes conventions
+
+## Decision
+
+TL;DR:
+
+```
+primer-react/
+├─ src/
+│  ├─ Breadcrumbs/
+│  │  ├─ index.ts                    // Just re-exporting?
+│  │  ├─ Breadcrumbs.tsx             // Primary component
+│  │  ├─ BreadcrumbsItem.tsx         // Subcomponent (include parent component name to increase findability in most IDEs)
+│  │  ├─ Breadcrumbs.mdx             // Documentation. Always .mdx, not .md
+│  │  ├─ Breadcrumbs.stories.tsx
+│  │  ├─ Breadcrumbs.test.tsx        // Unit tests
+│  │  ├─ Breadcrumbs.types.test.tsx  // Type tests
+│  │  ├─ Breadcrumbs.yml             // Component metadata (Possible future)
+│  │  └─ __snapshots__/
+┆  ┆
+```
+
+### Rules
+
+- Every component should have its own PascalCased directory directly under `src/`
+- Subcomponents should be properties of the exported parent component (e.g., `Breadcrumbs.Item`)
+- Replacements of existing components should use an incrementing number (e.g., `Breadcrumbs2` rather than `NewBreadcrumbs`)
+
+## Implementation
+
+- [ ] Migrate components into this structure
+- [ ] Set up tests for the component structure
+- [ ] Mark this ADR as adopted