chore(ignored/boilerplate files): minor cleanup

[chalin]

(1) We are ignoring all style files including the master styles.css:

> cd .../_examples
> git ls-files --ignored --exclude-standard | grep 'styles*.css'
styles.css

We don't want to do that. Now we only ignore styles.css in subdirectories. Cf. #894.

(2) The gulp target copy-example-boilerplate is also copying ts boilerplate into dart folders. This is fixed.

(3) Boilerplate files are now chmod -w so as to avoid that they be edited by mistake. Both of the following have been tested after these changes and continue to work as expected:

gulp add-example-boilerplate
gulp remove-example-boilerplate

(4) Entries in public/docs/_examples/.gitignore corresponding to boilerplate files have been moved to the top.

(5) Duplicate entries in public/docs/_examples/.gitignore relative to the top-level .gitignore have been removed.

(6) Entries in public/docs/_examples/.gitignore that are global in nature have been moved to the top-level .gitignore.

[wardbell]

LGTM. I am merging it

I'm prepared to see how life is with

Boilerplate files are now chmod -w

Sometimes I alter them for local purposes. But I see that could be a source of error. We can always roll this particular change back if it proves too onerous.

[chalin]

Btw, chmod -w doesn't prevent us from editing the files (I sometimes make local edits to); most modern editors/IDEs like Emacs and VSCode will warn the developer "This file is read-only, do you still want to save your changes?" and save changes to a read-only file if that is what the developer wants.