Add basic plugin developer docs

Closes #521

There's already a fair number of plugin examples which exist, and I don't
ever see taking all of them and merging it into a single document, so this
is probably the best that there will be for plugin development. I
expect that plugin development requires sufficient understanding of
how TypeDoc works that even the best guides won't allow people who haven't
read all of that material to do anything non-trivial anyways...

internal-docs/plugins.md

@@ -0,0 +1,48 @@
+# Writing a TypeDoc Plugin
+
+TypeDoc supports plugins which can modify how projects are converted, how converted symbols
+are organized, and how they are displayed, among other things. Plugins are Node modules which
+export a single `load` function that will be called by TypeDoc with the [Application] instance
+which they are to be attached to. Plugins should assume that they may be loaded multiple times
+for different applications, and that a single load of an application class may be used to convert
+multiple projects.
+
+Plugins may be either ESM or CommonJS.
+
+```js
+// @ts-check
+import td from "typedoc";
+/** @param {td.Application} app */
+export function load(app) {
+    // todo: Add event listeners to app, app.converter, etc.
+    // this function may be async
+}
+```
+
+Plugins affect TypeDoc's execution by attaching event listeners to one or many events that will be
+fired during conversion and rendering. Events are available on the [Application], [Converter],
+[Renderer], and [Serializer]/[Deserializer] classes. There are static `EVENT_*` properties on those
+classes which describe the available events.
+
+The best way to learn what's available to plugins is to browse the docs, or look at the source code
+for existing plugins. There is a list of currently supported plugins at https://typedoc.org/guides/plugins/
+
+TypeDoc also provides several control hooks for plugins to change it's behavior, they are described
+by the [third party symbols](./third-party-symbols.md) and [custom themes](./custom-themes.md) documents.
+
+Plugins which are configurable can add custom options with `app.options.addDeclaration`. [typedoc-plugin-mdn-links]
+has an example of the recommended way of doing this.
+
+To figure out what events a plugin should listen to, it is useful to have a high level understanding of how
+TypeDoc works. The [development page](https://typedoc.org/guides/development/) on the website describes this.
+
+If you have specific questions regarding plugin development, please open an issue or ask in the
+[TypeScript Discord] #typedoc channel.
+
+[Application]: https://typedoc.org/api/classes/Application.html
+[Converter]: https://typedoc.org/api/classes/Converter.html
+[Renderer]: https://typedoc.org/api/classes/Renderer.html
+[Serializer]: https://typedoc.org/api/classes/Serializer.html
+[Deserializer]: https://typedoc.org/api/classes/Deserializer.html
+[typedoc-plugin-mdn-links]: https://github.com/Gerrit0/typedoc-plugin-mdn-links/blob/main/src/index.ts
+[TypeScript Discord]: https://discord.gg/typescript