The Introduction to the buildsystem page still mentions this parameter
in case it needs to be changed, but in most situations, the default
value of `(number of CPU threads) - 1` does a good job.
- Use the long name for the `--editor` option to make it more obvious.
- Document how to run a project directly while debugging.
- Put comments above the line to avoid horizontal overflow.
This removes the link to the KDevelop setup video tutorial as it's
now outdated.
IDE screenshots were also optimized losslessly using
`oxipng -o6 --strip --zopfli`.
See #3485.
