Docs: update tooling and outdated/broken content

* Update docs build config
  * Specify Sphinx and theme versions in docs/requirements.txt
    (tox had been using latest; RTD had pinned older versions)
  * Update docs builds for Python 3 and recent Sphinx
  * Remove obsolete sphinx-rtd-theme patches
  * Add .readthedocs.yml
  * Move some JS to async (now that Sphinx supports that)
  * Fix version-alert.js for async loading
* Remove docs versionadded/changed info older than v3.0
* Fix broken GitHub issue links in changelog (never worked?)
* Add setup long_description_content_type
* Drop poorly maintained AUTHORS.txt 
  (just point to GitHub contributors page)

.readthedocs.yml

@@ -0,0 +1,16 @@
+# Read the Docs configuration file
+# See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
+
+version: 2
+
+sphinx:
+  configuration: docs/conf.py
+  builder: dirhtml
+
+# Additional formats to build:
+formats: all
+
+python:
+  version: 3.7
+  install:
+    - requirements: docs/requirements.txt