Release rinja v0.3.6+deprecated

[Kijewski]

• Create a branch rinja-v0.3.x.
• Update the book to contain a single page leading to the stable URL of https://rinja.readthedocs.io/en/latest/upgrading.html.
• Release rinja v0.3.6+deprecated containing a deprecation notice in the readme and docs.

Adding #![deprecated] to the module does work, but not as expected. The console will be spammed with hundreds of messages, because the deprecation message is not posted once, but for every import, every method call, and so on.

I added a branch rinja-v0.3.x. It contains docs how to update, but no warning messages of any kind.