Docs: update tooling and outdated/broken content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)

AUTHORS.txt

@@ -1,31 +1,5 @@
 Anymail
 =======
 
-Mike Edmunds
-Calvin Jeong
-Peter Wu
-Charlie DeTar
-Jonathan Baugh
-Noel Rignon
-Josh Kersey
-
-
-Anymail was forked from Djrill, which included contributions from:
-
-Kenneth Love
-Chris Jones
-Mike Edmunds
-ArnaudF
-Théo Crevon
-Rafael E. Belliard
-Jared Morse
-peillis
-José Padilla
-Jens Alm
-Eric Hennings
-Michael Hobbs
-Sameer Al-Sakran
-Kyle Gibson
-Wes Winham
-nikolay-saskovets
-William Hector
+Please see https://github.com/anymail/django-anymail/graphs/contributors
+for the complete list of Anymail contributors.