Fix RTDs compatibility with custom domains (#328)

* Fix RTDs compatilibity with custom domain

* Update conf.py

* Update conf.py

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

docs/conf.py

@@ -1,13 +1,19 @@
 #!/usr/bin/env python
 # -*- coding: utf-8 -*-
 
+import os
+
 from pkg_resources import DistributionNotFound, get_distribution
 
 try:
     __version__ = get_distribution("exoplanet").version
 except DistributionNotFound:
     __version__ = "unknown version"
 
+# Support canonical URL
+html_baseurl = os.environ.get("READTHEDOCS_CANONICAL_URL", "")
+if os.environ.get("READTHEDOCS", "") == "True":
+    html_context = {"READTHEDOCS": True}
 
 # General stuff
 extensions = [