Process description user manual is outdated

[rupertoverall]

User manual should be checked for conformity to the new updated specification.

The (outdated) user manual is currently found in two distinct directories:

1. https://github.com/sbgn/sbgn.github.io/tree/master/downloads/usermanual [The one linked from the SBGN website]

2. https://github.com/sbgn/process-descriptions/tree/main/user_manual [A note suggests removing this directory]