Add chevron arrow to edge-arrows documentation

[MarkBoyes]

The chevron arrow was added to cytoscape.js in 2018 but has not been included in the online demo.

Associated issues: #3148

The PR adds the arrow: "none" seems like a special case, so I have left "none" at the end of the page and made chevron the penultimate example.

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[maxkfranz]

This is a great catch, Mark.

It is important to have the 'none' example if the demo is to be exhaustive. Would you add the 'none' example back in?

[MarkBoyes]

Unless I fouled up the commit, the none case is still there as the new e11 joining n22 and n23

[maxkfranz]

It looks like I misread the diff. My mistake.

I'll set aside some time in the next day or so to try out a build of the documentation and get this merged in.

Thanks for the PR.