add edit button in site breadcrumb

[hboutemy]

like done on https://maven.apache.org/

[ilgrosso]

Honestly, I don't like much the idea of ditching gitbox.apache.org for GitHub...

[ilgrosso]

@hboutemy let's discuss this proposed change in the dev@syncope mailing list, please

[hboutemy]

I'll have a look at your ML
I understand your relunctance to GH: yes, there is a choice to do to ease access to documentation source or keep everything pointing to Gitbox
don't hesiatte to drop this PR: there was not much effort in it

[ilgrosso]

Thank you.
I'll close this for now: feel free to reach up our dev mailing list to further discuss this topic, we can always re-open later.