[Docs] Replace recommonmark with MyST parser

We used recommonmark so that we can use markdown in sphinx but it is
buggy and now so switch to better supported MyST-parser.

* Fixed incorrect heading warnings in markdown.
* Added sphinx toctree to markdown using directive as required by MyST.
* Upgraded Sphinx to 4.3

Ref: https://myst-parser.readthedocs.io

docs/source/contributing/index.md

@@ -3,7 +3,11 @@
 Deluge is an open-source project, and relies on its community of users to keep
 getting better.
 
-- [Code contributions](code.md)
-- [Running tests](testing.md)
-- [Documentation contributions](documentation.md)
-- [Translation contributions](translations.md)
+```{toctree}
+:titlesonly:
+
+code
+testing
+documentation
+translations
+```

docs/source/contributing/testing.md

@@ -38,7 +38,7 @@ Running the tests for a specific plugin (requires [pytest](https://pypi.python.o
 
 All the tests for Deluge can be run using [Tox](https://pypi.python.org/pypi/tox)
 
-#### See available targets:
+### See available targets:
 
     tox -l
     py27
@@ -46,11 +46,11 @@ All the tests for Deluge can be run using [Tox](https://pypi.python.org/pypi/tox
     lint
     docs
 
-#### Run specific test:
+### Run specific test:
 
     tox -e py3
 
-#### Verify code with pre-commit:
+### Verify code with pre-commit:
 
     tox -e lint