Block Directory: Add developer docs to readme

[ryelle]

Description

This PR adds a brief overview for how the block directory works, and injects the actions & selectors docs into the readme for better visibility.

I'm unsure if other sections need documentation too — I skipped over the components directory, since those files might change wholesale with #25521, but maybe the plugins directory could use a README to explain what each folder is used for?

How has this been tested?

You can view the updated readme here

[github-actions]

Size Change: +1.67 kB (0%)

Total Size: 1.17 MB

Filename	Size	Change
build/api-fetch/index.js	3.35 kB	+11 B (0%)
build/block-directory/index.js	8.53 kB	+1 B
build/block-editor/index.js	128 kB	+46 B (0%)
build/block-editor/style-rtl.css	11.1 kB	-2 B (0%)
build/block-editor/style.css	11.1 kB	-1 B
build/block-library/editor-rtl.css	8.6 kB	+37 B (0%)
build/block-library/editor.css	8.6 kB	+37 B (0%)
build/block-library/index.js	135 kB	+553 B (0%)
build/block-library/style-rtl.css	7.61 kB	+9 B (0%)
build/block-library/style.css	7.6 kB	+12 B (0%)
build/blocks/index.js	47.5 kB	-1 B
build/components/index.js	167 kB	+22 B (0%)
build/core-data/index.js	12 kB	-1 B
build/data-controls/index.js	1.27 kB	+2 B (0%)
build/data/index.js	8.43 kB	+4 B (0%)
build/edit-navigation/index.js	10.4 kB	-48 B (0%)
build/edit-post/index.js	306 kB	+1 B
build/edit-post/style-rtl.css	6.25 kB	+10 B (0%)
build/edit-post/style.css	6.24 kB	+10 B (0%)
build/edit-site/index.js	20.3 kB	+586 B (2%)
build/edit-site/style-rtl.css	3.51 kB	+206 B (5%)	🔍
build/edit-site/style.css	3.51 kB	+204 B (5%)	🔍
build/editor/index.js	45.4 kB	-57 B (0%)
build/editor/style-rtl.css	3.83 kB	+26 B (0%)
build/editor/style.css	3.82 kB	+24 B (0%)
build/element/index.js	4.44 kB	-8 B (0%)
build/rich-text/index.js	13.7 kB	-10 B (0%)

ℹ️ View Unchanged

Filename	Size	Change
build/a11y/index.js	1.14 kB	0 B
build/annotations/index.js	3.52 kB	0 B
build/autop/index.js	2.72 kB	0 B
build/blob/index.js	620 B	0 B
build/block-directory/style-rtl.css	943 B	0 B
build/block-directory/style.css	942 B	0 B
build/block-library/theme-rtl.css	741 B	0 B
build/block-library/theme.css	741 B	0 B
build/block-serialization-default-parser/index.js	1.78 kB	0 B
build/block-serialization-spec-parser/index.js	3.1 kB	0 B
build/components/style-rtl.css	15.5 kB	0 B
build/components/style.css	15.5 kB	0 B
build/compose/index.js	9.42 kB	0 B
build/date/index.js	31.9 kB	0 B
build/deprecated/index.js	772 B	0 B
build/dom-ready/index.js	568 B	0 B
build/dom/index.js	4.42 kB	0 B
build/edit-navigation/style-rtl.css	868 B	0 B
build/edit-navigation/style.css	871 B	0 B
build/edit-widgets/index.js	17.5 kB	0 B
build/edit-widgets/style-rtl.css	2.8 kB	0 B
build/edit-widgets/style.css	2.8 kB	0 B
build/editor/editor-styles-rtl.css	492 B	0 B
build/editor/editor-styles.css	493 B	0 B
build/escape-html/index.js	733 B	0 B
build/format-library/index.js	7.49 kB	0 B
build/format-library/style-rtl.css	547 B	0 B
build/format-library/style.css	548 B	0 B
build/hooks/index.js	1.74 kB	0 B
build/html-entities/index.js	621 B	0 B
build/i18n/index.js	3.55 kB	0 B
build/is-shallow-equal/index.js	709 B	0 B
build/keyboard-shortcuts/index.js	2.39 kB	0 B
build/keycodes/index.js	1.85 kB	0 B
build/list-reusable-blocks/index.js	3.02 kB	0 B
build/list-reusable-blocks/style-rtl.css	476 B	0 B
build/list-reusable-blocks/style.css	476 B	0 B
build/media-utils/index.js	5.12 kB	0 B
build/notices/index.js	1.69 kB	0 B
build/nux/index.js	3.27 kB	0 B
build/nux/style-rtl.css	671 B	0 B
build/nux/style.css	668 B	0 B
build/plugins/index.js	2.44 kB	0 B
build/primitives/index.js	1.34 kB	0 B
build/priority-queue/index.js	789 B	0 B
build/redux-routine/index.js	2.85 kB	0 B
build/server-side-render/index.js	2.61 kB	0 B
build/shortcode/index.js	1.7 kB	0 B
build/token-list/index.js	1.24 kB	0 B
build/url/index.js	4.06 kB	0 B
build/viewport/index.js	1.74 kB	0 B
build/warning/index.js	1.13 kB	0 B
build/wordcount/index.js	1.17 kB	0 B

_{compressed-size-action}

[StevenDufresne]

Awesome, this is a nice improvement.

Thanks for this! 👏 .