chore(infrastructure): deployment should tag deprecated keyboard help php files

[mcdurdin]

But, as a separate project, it might be good to have the keyboard help deployment add a DEPRECATED.md to keyboard help folders that are deprecated (and also to any keyboard help folders it finds that don't have a corresponding currently-published keyboard), and then update help.keyman.com/keyboard/index.php to skip those for the default help index, and list them only in a separate 'deprecated keyboards' index.

Originally posted by @mcdurdin in #1859 (comment)

[LornaSIL]

@mcdurdin do you want me to add a DEPRECATED.md file into all the help folders where the keyboard is deprecated?
If so, can the contents be the same as the DEPRECATED.md file in the root folder of the keyboard?

[mcdurdin]

@LornaSIL, no I think we can automate this, which will ensure that we catch all the deprecated content.