[DOCS] Fix problem with Edit in github link

The link is created with docs in it, even if the mkdocs.yml file
mentions doc as the documentation folder. I am not aware if there is
any other dependency in the name doc, I have renamed it to docs and
changed it also in the mkdocs.yml file. It seems docs is hardcoded in
the readthedocs theme :(

docs/modules/dart.md

@@ -0,0 +1,23 @@
+# Prelude Dart
+
+!!! Note
+
+    This module builds on top of the lsp-module.
+
+The dart module is powered by [lsp-dart](https://emacs-lsp.github.io/lsp-dart/).
+
+Some features it provides:
+
+- Flutter integration
+- Tree views
+- Run pub
+- Debug
+- Run tests
+
+Some essential commands:
+
+- `lsp-dart-pub-get` - Run pub get or flutter pub get on project root.
+- `lsp-dart-pub-upgrade` - Run pub upgrade or flutter pub upgrade on project root.
+- `lsp-dart-pub-outdated` - Run pub outdated or flutter pub outdated on project root.
+- `lsp-dart-run-all-tests` - Run all tests from project.
+- `lsp-dart-run-test-file` - Run all tests from current test buffer.