Add an example of underline to the editor help

[matejak]

I have found out that _underline_ resulting in actual underline is supported, but not documented in the help.

So I have added this to the help page code by copy-pasting and altering corresponding sections, i.e. I have not tested the PR. Please let me know if it is needed, and if there is a corresponding guide that would assist in testing.

[NGPixel]

While it is supported, it is not enabled by default. Most users would look at the help, try it and wonder why it doesn't work.

[matejak]

What do you mean by supported but not enabled? When I try the _underline_ syntax, it works for me in wikijs.

[NGPixel]

It works because you enabled underline under admin > rendering.

The default for new installations is off.

[matejak]

I see, it was probably an old installation. So if this is an unwanted feature, then it would make sense to close this, and if it is a meaningful feature, this PR should be probably expanded to the state when everything is supported, documented and working at the same time.