Some bits for RTD

Author: michaelpj

Diff for: .readthedocs.yaml

@@ -0,0 +1,9 @@
+version: 2
+
+sphinx:
+  configuration: docs/conf.py
+
+python:
+  version: 3.9
+  install:
+    - requirements: docs/requirements.txt

Diff for: docs/requirements.txt

@@ -0,0 +1,4 @@
+Sphinx==4.0.2
+sphinx-rtd-theme==0.5.2
+myst-parser==0.15.1
+docutils<0.17

Diff for: flake.nix

@@ -191,7 +191,7 @@
             sha256 = "0rkrsvvqr6g2p3v5vq88jhfp5sd0r1jqjh3vc5y26jn30z8s4fkz";
           };
         });
-        pythonWithPackages = pkgs.python3.withPackages (ps: [ps.sphinx myst-parser sphinx_rtd_theme]);
+        pythonWithPackages = pkgs.python3.withPackages (ps: [ps.sphinx myst-parser sphinx_rtd_theme ps.pip]);
 
         # Create a development shell of hls project
         # See https://github.com/NixOS/nixpkgs/blob/5d4a430472cafada97888cc80672fab255231f57/pkgs/development/haskell-modules/make-package-set.nix#L319