Document the hand shape font

[Artoria2e5]

The files signalphabets.tsv and forms.csv contain Private Use Area characters like . Such characters make no sense unless a specific font is used. Judging from the screenshots in the paper, they are supposed to represent hand shapes. Would you mind documenting what font is used, and from where it is available?

PS: Unicode actually does have an area for sign language hand shapes, in the form of a SignWriting block (code chart). It may be better for interoperability if the database is converted sometime, but I do understand that it takes work to adapt between Hamnosys and SW models.

[LinguList]

Since Hamnosys has also equivalents-short-hands for the symbols, we can also adjust the custom-unicode range with identifiers that are human-readable. Doing this is rather straightforward.