Fixes #2840.

Since the behaviour of the current script was not so obvious, I set the requirements as follows and edited the script therefore.

If the layout.html was deleted:

• throw error "Error: the layout "layout.html" file is required!"

If the layout.html was updated:

• loads the layout file and regenerates all html files in dist

If a navigation file (nav-guides.md|nav-methods.md) was deleted:

• throw error "Error: the navigation file "nav-guides.md" is required!"

If a navigation file was updated:

• loads the navigation files and regenerates all pages in dist

If a markdown file was deleted:

• deletes the html page from dist

If a markdown file was updated:

• regenerates the html page in dist

If a markdown file was added:

• generates the html page in dist

If an asset file was deleted:

• deletes the asset file from dist

If an asset file was updated:

• updates the asset file in dist

If an asset file was added:

• adds the asset file to dist