docs: Add docs docs

docs/README.md

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+# `transcoder` docs
+
+This buildchain is a self-contained build system for the `transcoder` documentation.
+
+## Overview
+
+The buildchain uses [VitePress](https://vitepress.dev/) via a Docker container to facilitate writing the docs as [markdown](https://vitepress.dev/guide/markdown), linting them via [textlint](https://textlint.github.io/), building them as HTML files with bundled assets, and publishing them automatically via a [GitHub action](https://docs.github.com/en/actions).
+
+It also uses a [Rollup](https://rollupjs.org/) [sitemap plugin](https://github.com/aminnairi/rollup-plugin-sitemap) to generate a `sitemap.xml` for the generated docs.
+
+The markdown sources for the docs and assets are in the `docs/docs/` directory.
+
+The built distribution docs are created via the `build-and-deploy-docs.yaml`
+
+## Prerequisites
+
+To run the buildchain for development purposes:
+
+- You must have [Docker Desktop](https://www.docker.com/products/docker-desktop/) (or the equivalent) installed
+
+## Commands
+
+This buildchain uses `make` as an interface to the buildchain. The following commands are available from the `buildchain/` directory:
+
+- `make image-build` - Build the Docker image & run `npm install`. This command must be run once before using the Docker container.
+- `make dev` - Start Vite HMR dev server while writing/editing the docs. Click on the link displayed in the terminal to open the docs up
+- `make lint` - Run `textlint` on the docs, reporting on any errors and warnings
+- `make fix` - Run `textlint` on the docs, automatically fixing any errors, and reporting any warnings
+- `make clean` - Remove `node_modules/` and `package-lock.json` to start clean (need to run `make image-build` after doing this, see below)
+- `make npm XXX` - Run an `npm` command inside the container, e.g.: `make npm run lint` or `make npm install`
+- `make ssh` - Open up a shell session into the buildchain Docker container
+- `make build` - Do a local distribution build of the docs; normally not needed since they are built & deployed via GitHub action
+
+## Docs versioning
+
+Each major version of the plugin corresponds to a major version of Craft.
+
+Each major version of the plugin has separate documentation that needs to be updated when changes span plugin versions.
+
+The latest version of the docs that correspond to the latest version of the plugin is always the root of the docs tree, with older versions appearing in sub-directories:
+
+```
+│ index.html
+├── v4
+│   └── index.html
+├── v3
+│   └── index.html
+```
+
+The docs are entirely separate, but linked to eachother via a version menu, configured in the `docs/docs/.vitepress/config.ts` file.
+
+## Algolia Docsearch
+
+The docs uses [Algolia Docsearch](https://docsearch.algolia.com/) to index them, and allow for easy searching via a search field with auto-complete.
+
+Algolia Docsearch is configured in the `docs/docs/.vitepress/config.ts` file.
+
+## textlint
+
+The buildchain uses [textlint](https://textlint.github.io/) to automatically fix errors on build, and also issue writing style warnings.
+
+`textlint` automatically uses any rules added to the `docs/package.json` file, which can be overridden or customized via the `docs/.textlintrc.js` file.
+
+See the [textlint docs](https://textlint.github.io/docs/getting-started.html) for details.
+
+## Overriding environment variables
+
+The `Makefile` contains sane defaults for most things, but you can override them via environment variables if you need to.
+
+For instance, if you want to change the `DOCS_DEST` environment variable to change where `make build` builds the docs locally, you can set it before running any buildchain `make` commands:
+```bash
+env DOCS_DEST=../path/to/some/dir make build
+```
+...or use any other method for [setting environment variables](https://www.twilio.com/blog/how-to-set-environment-variables.html). This environment variable needs to be set in the shell where you run the buildchain's various `make` commands from, so setting it in your project's `.env` file won't work.