Update link in the readme and setup (#12788)

Author: Max Jones

README.md

@@ -20,7 +20,7 @@ We are always happy to answer questions! Here are some good places to ask them:
 - for general questions about Python typing, try [typing discussions](https://github.com/python/typing/discussions)
 
 If you're just getting started,
-[the documentation](https://mypy.readthedocs.io/en/stable/introduction.html)
+[the documentation](https://mypy.readthedocs.io/en/stable/index.html)
 and [type hints cheat sheet](https://mypy.readthedocs.io/en/stable/cheat_sheet_py3.html)
 can also help answer questions.
 

setup.py

@@ -210,7 +210,7 @@ def run(self):
       include_package_data=True,
       project_urls={
           'News': 'http://mypy-lang.org/news.html',
-          'Documentation': 'https://mypy.readthedocs.io/en/stable/introduction.html',
+          'Documentation': 'https://mypy.readthedocs.io/en/stable/index.html',
           'Repository': 'https://github.com/python/mypy',
       },
       )