@import "notice"; and @import "bootstrap"; fails when running `./m css`

[RogerHaase]

Documenting need to fixup missing imports after #1908:

Convert Basic theme from Boostrap 3 to Boostrap 4 and use sass instead of lessc (also fixes #1770) #1908

If you get the @import "notice"; and @import "bootstrap"; errors running ./m css then do:

1. point your browser to https://github.com/twbs/bootstrap/tree/v4.5.3/scss (optional)
2. modify the browser url to https://ssgithub.com/twbs/bootstrap/tree/v4.5.3/scss
3. click the download button to save a zip file to a convenient location
4. unzip the file to xstatic/pkg/bootstrap/data creating a scss directory as a sibling to the existing css and js directories
5. rerun ./m css

[roland-ruedenauer]

In #1908 I mentioned:

A new release of XStatic-Bootstrap should provide the required scss files.

Unfortunately the process of releasing updated XStatic packages got stuck and the boostrap SCSS files are still missing now months later.

My suggestion made in #1895 would still be an (imo good) option to solve this obstacle.

[RogerHaase]

docs updated https://moin-20.readthedocs.io/en/latest/devel/development.html#add-more-tools-exercise-tools