Set static path to empty

It is not needed since we have no static assets and results in a warning
on GitHub Actions: "WARNING: html_static_path entry '_static' does not exist"

See readthedocs/readthedocs.org#1776 (comment)

Author: MisanthropicBit

docs/conf.py

@@ -70,7 +70,7 @@
 # Add any paths that contain custom static files (such as style sheets) here,
 # relative to this directory. They are copied after the builtin static files,
 # so a file named "default.css" will overwrite the builtin "default.css".
-html_static_path = ['_static']
+html_static_path = []
 
 base_url = 'https://github.com/{github_user}/{github_repo}/blob/{branch}'\
     .format(branch='master', **html_theme_options)