[INTERNAL] grunt: update to new version of "grunt-openui5" / watch mode

- updated "grunt-openui5" dependency
- changed configuration accordingly
- enabled serve --watch mode
- updated related docs

Change-Id: I76d5b517ae0e2fa930e72dec49822f24b2a8d8ac
(cherry picked from commit 3eb4529d7462e2c990b44d2ef143718dc80700c0)
CR-Id: 012003146900009282252014
Merge "[INTERNAL] grunt: update to new version of "grunt-openui5" / watch mode" into rel-1.26