Release process: changelog generation and docs (#3446)

* Remove extra pyprojects and update changelog docs

* Add script for rendering the changelog

* Add docs for how to release the spec

* Move legacy changelogs out of the way

changelogs/identity_service/pyproject.toml

@@ -1,35 +0,0 @@
-[tool.towncrier]
-    filename = "../identity_service.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 = "removal"
-        name = "Removed Endpoints"
-        showcontent = true
-
-    [[tool.towncrier.type]]
-        directory = "feature"
-        name = "Backwards Compatible Changes"
-        showcontent = true
-
-    [[tool.towncrier.type]]
-        directory = "clarification"
-        name = "Spec Clarifications"
-        showcontent = true