(Installation.md): add `data` (used by `sf project:disable` commands)
(Installation.md): add `data` (used by `sf project:disable` commands)

The only use we have for this `data/` folder is sf's builtin
"website maintenance" feature, using CLI

`sf project:disable koohii (env)` etc.

Sf creates a temporary "lock" file in the data/ folder. It won't create
the folder if it doesn't exist.

fabd/kanji-koohii,fabd/kanji-koohii,fabd/kanji-koohii,fabd/kanji-koohii,fabd/kanji-koohii,fabd/kanji-koohii