Upgrading to Airflow 2.0 doc (#12930)

This is based heavily on the UPGRADING_to_2.0.md file. The intent here is
to make user facing and available as part of the Airflow documentation.
This is still a work in progress.

Co-authored-by: Kaxil Naik <kaxilnaik@gmail.com>
Co-authored-by: Jarek Potiuk <jarek@potiuk.com>
Co-authored-by: Daniel Imberman <daniel.imberman@gmail.com>
Co-authored-by: Ash Berlin-Taylor <ash_github@firemirror.com>

docs/apache-airflow/index.rst

@@ -79,6 +79,8 @@ Content
     license
     start
     installation
+    upgrading-to-2
+    upgrade-check
     tutorial
     tutorial_taskflow_api
     howto/index

docs/apache-airflow/upgrade-check.rst

@@ -21,6 +21,8 @@ Upgrade Check Script
 
 .. contents:: :local:
 
+.. _upgrade-check:
+
 Getting the Airflow Upgrade Check Package
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