Update conf.py from git tag, fix sphinx warnings (#5716)

Fixes #5671

Implements https://protips.readthedocs.io/git-tag-version.html

Fix a myriad of minor Sphinx warnings generated in the docs.

package/README.md

@@ -138,7 +138,3 @@ The following points assume work in a direct clone of the repository, and not in
     * In main README.md:
 
         - in "Latest release" section, change version number in the readthedocs link to the version which was just released, and verify that all links work.
-        
-    * In doc/conf.py
-   
-        - update version and release to the *next* milestone

package/package_esp8266com_index.template.json

@@ -299,4 +299,4 @@
          "name": "esp8266"
       }
    ]
-}
+}