Fixes https://github.com/matrix-org/matrix-doc/issues/1499
This is done by using magic variables in the RST. The magic variables are generated based on the substitutions available, making them available for use at build-time.
Magic variables were chosen because it allows people to continue working on the spec and release process without having to worry about removing a chunk of text from the top of the file. Originally, this was attempted by using jinja2 if-statements, however the substitutions are replaced *after* the template is executed, so the condition would never match.
The format of the variable is to make the templating happy. Using colons or percent signs results in the templator thinking something else is going on, and then complaining about format.
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.
