fix: read the docs build works again (#496)

proof of working build -
https://readthedocs.org/projects/testcontainers/builds/23851186/

this is the one i created - "testcontainers" - the one we actually link
to is "testcontainers-python"

not sure how to update to re-run that one, which is what we want to do
(and i will either rename or delete the one i just made).

Author: alexanderankin

conf.py

@@ -48,7 +48,7 @@
 source_suffix = ".rst"
 
 # The master toctree document.
-master_doc = "INDEX"
+master_doc = "index"
 
 # General information about the project.
 project = "testcontainers"