r0.1.0 release of the Push Gateway specification

Because this is the first release, it has several moving parts to it:
* The version variables have been defined.
* The towncrier changelog has been prepared for future modifications.
* The templating has been updated to better support future versions of the specification.
* A release process document has been created.

changelogs/push_gateway/newsfragments/.gitignore

@@ -0,0 +1 @@
+!.gitignore

changelogs/push_gateway/pyproject.toml

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+[tool.towncrier]
+    filename = "../push_gateway.rst"
+    directory = "newsfragments"
+    issue_format = "`#{issue} <https://github.com/matrix-org/matrix-doc/issues/{issue}>`_"
+    title_format = "{version}"
+    
+    [[tool.towncrier.type]]
+        directory = "breaking"
+        name = "Breaking Changes"
+        showcontent = true
+    
+    [[tool.towncrier.type]]
+        directory = "deprecation"
+        name = "Deprecations"
+        showcontent = true
+    
+    [[tool.towncrier.type]]
+        directory = "new"
+        name = "New Endpoints"
+        showcontent = true
+    
+    [[tool.towncrier.type]]
+        directory = "feature"
+        name = "Backwards Compatible Changes"
+        showcontent = true
+    
+    [[tool.towncrier.type]]
+        directory = "clarification"
+        name = "Spec Clarifications"
+        showcontent = true