Handbook: Adjust style of H5 headers

See the H5 headers in Note input: Computer keyboard (Note duration etc.).

The H5 (#####) headers in the handbook need some adjustment:

1. Reduce space under the H5 header so that it is comparable to that under H3 and H4 headers.
2. Increase size a little (and adjust font-weight?) to help to distinguish H5 headers from (1) ordinary bold text and (2) H4 headers.