Devdocs: serve the content of README.md files from the search index module

[jsnajdr]

While working on #38190 and updating the devdocs/bin/generate-devdocs-index script, I discovered that in addition to the search index, we also store the text content of the indexed files in the generated search-index.js module. It's used to generate snippets (that contain the searched term) for the search results. This text content is plain text with all markup stripped.

If we stored the raw Markdown content to this module, too, we could use it to serve the README.md files content in the /devdocs/service/content endpoint. This PR implements exactly that. Currently it's served from the Calypso source tree that needs to be present at runtime.

I think this is a better and more elegant solution than moving the Markdown content to the client webpack bundle that @ockham proposes in #35833. Mainly because it doesn't inflate the webpack manifest with 800+ new chunks.

It also makes the Calypso server more self-contained: soon we'll need only the build/ and public/ directory to run the Calypso server, and not the whole source tree.

Changes proposed in this Pull Request

1. Move the generating script to the /bin directory and give it a .js suffix. That's a better location and the .js extension will make it Prettier-ed and ESLint-ed.
2. In package.json, make sure that the build-devdocs:* tasks are finished before build-server starts. build-server uses the files generated by build-devdocs:*. Until now, they were running in parallel and there was a race condition present.
3. In package.json, simplify and rename the build-devdocs:index task. The script doesn't need any special environment variables and can be run with a simple node command.
4. In the generate-devdocs-index.js script, format it with Prettier and fix ESLint errors. And also store the raw content as a content property of the generated document records.
5. Finally, in the implemenation of the /devdocs/service/endpoint, use searchIndex.documents[ idx ].content to serve the raw content. And remove the FS access.

Testing instructions

• test that the README search at /devdocs works correctly
• test that the <ReadmeViewer> component that's used across devdocs shows the README documents as expected.

[matticbot]

Test live: https://calypso.live/?branch=devdocs/serve-content-from-search-index

[matticbot]

This PR does not affect the size of JS and CSS bundles shipped to the user's browser.

Generated by performance advisor bot at iscalypsofastyet.com.

[ockham]

Nice! This has a much smaller footprint than mine!

Unfortunately, none of the docs in devdocs/docs/ are currently working (The Calypso Guide, Accessibility, Color, Icons); same for Contributing (.github/CONTRIBUTING.md -- maybe see #36167?)

(Typography, which is in devdocs/, does work, as doe component and blocks docs -- except for the ones that we've moved into packages/components, but that's a known and pre-existing issue.)

[jsnajdr]

Unfortunately, none of the docs in devdocs/docs/ are currently working

Oh, I misunderstood what path.relative() really does and didn't test the patch sufficiently 🙁 Fixed in f5aac84

except for the ones that we've moved into packages/components

Files in packages/ were not part of the search index until now -- that's fixed by this added line: https://github.com/Automattic/wp-calypso/pull/38784/files#diff-8434269b829266816be9c69918de4979R24

Displaying their content now works, too -- I don't see any regressions or bugs for packages/components/

All issues from the review should be addressed now.

[ockham]

Thanks @jsnajdr, this is working great now!