Add developer documentation

[adamziel]

Replaces #52

This PR aims to help new contributors get started by providing the necessary developer documentation. The proposed markdown file is a start but it needs more work before merging – feel free to jump in, comment, and update as you see fit.

The doc in this PR is all about the internal structure of this project. Documenting public-facing APIs doesn't make sense at this point as the project is still early, has no stable public API, and needs more contributors.

Related to #12

cc @dd32 @gziolo @StevenDufresne @eliot-akira

[eliot-akira]

This section of the PR description in #57 ("Monorepo") looks useful as reference.

The new packages:

* **php-wasm** – low-level WASM PHP primitives
  
  * A configurable PHP build pipeline for different targets (web, node.js, standalone)
  * A low-level `PHP` JavaScript class with `eval` for executing PHP code and FS utils like `writeFile` for runtime managing the
  * A `PHPServer` JavaScript class for dispatching HTTP requests – both to run the PHP files AND to download static files
  * A `PHPBrowser` JavaScript class to consume the above using an iframe

* **php-wasm-browser** – a high-level layer to efficiently run `php-wasm` in the browser
  
  * `service-worker` utilities to redirect the browser traffic to `PHPServer`
  * `worker-thread` utilities to offload the `PHPServer` to a separate process. Three backends are available: Iframe, Webworker, SharedWorker.
  * A messaging layer and setup helpers to connect the above.
  * Server utilities to serve the correct headers for the `wasm` files via `.htaccess`.

* **wordpress-wasm** – WordPress-specific WASM PHP bindings for web and node.js
  
  * The required WordPress-specific setup like constants and filters
  * `wp.data` bundling pipeline for the web configurable to bundle custom code
  * WordPress API to ease common tasks like login, install a plugin, start a block editor with specific settings and content
  * A `fetch`-based transport for HTTP requests
  * An example app demonstrating WordPress in the browser. Let's eventually extract it into a separate package or at least a directory.

[adamziel]

As I wrote these docs, many ideas stood out to me as overly complex. This PR got polluted with code refactoring, but so be it – there isn't much value in separating them at this point.

I'll go ahead and merge even though this PR could use some polish. There is almost no documentation at the moment and even unpolished docs can onboard new contributors .

Follow-up work:

• Iterate on the existing docs, fix typos, gather more feedback, reword confusing fragments.
• Generate a proper reference doc in a markdown format. It should include the top-level module documentation, function documentation, and the classes + their methods. I'm not sure which tool can handle that correctly.
• Add the missing wordpress-wasm package documentation
• Fill-in any remaining blanks