doc: Add README & Update Starlight

doc/README.md

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+## RIOT Documentation
+
+RIOT has various documentation sources in this folder, including:
+
+### Starlight / Guide Site
+
+Starlight is the main documentation site for RIOT, located at
+[guide.riot-os.org](https://guide.riot-os.org/),
+the `starlight` directory itself mostly only holds the source files for the site,
+excluding most guides which are stored within `guides`.
+
+If you wish to contribute new guides or find out more, please see
+[Creating Guides & How Starlight Works](https://guide.riot-os.org/misc/how_to_doc/).
+
+### Doxygen / API Site
+
+Doxygen is the home of all the API documentation,
+located at [api.riot-os.org](https://api.riot-os.org/).
+The `doxygen` directory holds the source files for the API documentation,
+which is generated from the RIOT source code (header and `doc.md` files) itself.
+
+New guides should not be added directly to the Doxygen documentation,
+but rather to the Starlight site, which can then link to the API documentation as needed,
+unless the guide is very specific to the API and would not fit well within the Starlight site.