Add rel7.3.0.rst landing page

R1/source/releases/rel7.3.0.rst

+.. _rel7.3.0:
+
+Varnish Cache 7.3.0
+===================
+
+* Source download `varnish-7.3.0.tgz </downloads/varnish-7.3.0.tgz>`_
+
+* SHA256=e2dbbb0ec270a90647c386866e6e226993aed46e48de751a72bb819737f14ae7
+
+Varnish Cache 7.3.0 is a regular bi-annual "fresh" release. It supersedes
+the :ref:`rel7.2.1` release.
+
+More information:
+
+* List of most important `Changes in 7.3 <https://varnish-cache.org/docs/7.3/whats-new/changes-7.3.html>`_
+* Help on `Upgrading to Varnish 7.3 <https://varnish-cache.org/docs/7.3/whats-new/upgrading-7.3.html>`_
+* `Full changes.rst entry for 7.3.0 <https://github.com/varnishcache/varnish-cache/blob/7.3/doc/changes.rst#varnish-cache-730-2023-03-15>`_
+
+For installation instructions including information about cloud images see
+`the Varnish Installation Manual </docs/trunk/installation/index.html>`_
+
+We recommend using the manual installation procedure available at:
+https://packagecloud.io/varnishcache/varnish73/install#manual