Docs: Remove Key-Cert authentication info (atlassian-api#682)

Documentation gave information about authentication with key and certificate.
This method cannot be found in the code and not in the Atlassian
documentation.

Therefore, the chapter is removed and the other authentication methods are
grouped with a new title.

Author: jensvog

docs/index.rst

@@ -49,45 +49,10 @@ Add a connection:
         username='admin',
         password='admin')
 
-Key/Cert Based authentication
------------------------------
+Other authentication methods
+----------------------------
 
-Add a connection using key/cert based authentication:
-
-.. code-block:: python
-
-    from atlassian import Jira
-    from atlassian import Confluence
-    from atlassian import Bitbucket
-    from atlassian import ServiceDesk
-    from atlassian import Xray
-
-    jira = Jira(
-        url='http://localhost:8080',
-        key='/path/to/key',
-        cert='/path/to/cert')
-
-    confluence = Confluence(
-        url='http://localhost:8090',
-        key='/path/to/key',
-        cert='/path/to/cert')
-
-    bitbucket = Bitbucket(
-        url='http://localhost:7990',
-        key='/path/to/key',
-        cert='/path/to/cert')
-
-    service_desk = ServiceDesk(
-        url='http://localhost:8080',
-        key='/path/to/key',
-        cert='/path/to/cert')
-
-    xray = Xray(
-        url='http://localhost:8080',
-        key='/path/to/key',
-        cert='/path/to/cert')
-
-Alternatively OAuth can be used:
+Further authentication methods are available. For example OAuth can be used:
 
 .. code-block:: python