Use sphinx version 2.4.4 for markdown links to work

Signed-off-by: Daniel Lim Wee Soong <weesoong.lim@gmail.com>

docs/environment.yml

diff --git a/docs/environment.yml b/docs/environment.yml
index c219d35..794b84c 100644
--- a/docs/environment.yml
+++ b/docs/environment.yml
@@ -14,8 +14,7 @@
 # ReadTheDoc dependencies
 - mock
 - pillow
-- sphinx
-- sphinx_rtd_theme
+- sphinx=2.4.4
 # Packages installed from PyPI
 - pip:
   - -r file:requirements.txt

docs/requirements.txt

diff --git a/docs/requirements.txt b/docs/requirements.txt
index fb96629..e8a9f52 100644
--- a/docs/requirements.txt
+++ b/docs/requirements.txt
@@ -1,10 +1,8 @@
 sphinx_materialdesign_theme
 
 docutils
-sphinx
 sphinx-autobuild
 
-breathe
 recommonmark
 sphinx-markdown-tables
 sphinxcontrib-napoleon