Añade módulo autónomo para integrar [mdbook]

website/doc/en/src/for_developers/README.md

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+# For Developers
+
+While `mdbook` is mainly used as a command line tool, you can also import the
+underlying library directly and use that to manage a book. It also has a fairly
+flexible plugin mechanism, allowing you to create your own custom tooling and
+consumers (often referred to as *backends*) if you need to do some analysis of
+the book or render it in a different format.
+
+The *For Developers* chapters are here to show you the more advanced usage of
+`mdbook`.
+
+The two main ways a developer can hook into the book's build process is via,
+
+- [Preprocessors](preprocessors.md)
+- [Alternative Backends](backends.md)
+
+
+## The Build Process
+
+The process of rendering a book project goes through several steps.
+
+1. Load the book
+    - Parse the `book.toml`, falling back to the default `Config` if it doesn't
+       exist
+    - Load the book chapters into memory
+    - Discover which preprocessors/backends should be used
+2. For each backend:
+   1. Run all the preprocessors.
+   2. Call the backend to render the processed result.
+
+
+## Using `mdbook` as a Library
+
+The `mdbook` binary is just a wrapper around the `mdbook` crate, exposing its
+functionality as a command-line program. As such it is quite easy to create your
+own programs which use `mdbook` internally, adding your own functionality (e.g.
+a custom preprocessor) or tweaking the build process.
+
+The easiest way to find out how to use the `mdbook` crate is by looking at the
+[API Docs]. The top level documentation explains how one would use the
+[`MDBook`] type to load and build a book, while the [config] module gives a good
+explanation on the configuration system.
+
+
+[`MDBook`]: https://docs.rs/mdbook/*/mdbook/book/struct.MDBook.html
+[API Docs]: https://docs.rs/mdbook/*/mdbook/
+[config]: https://docs.rs/mdbook/*/mdbook/config/index.html