apidocs explorer refers to non-existent file.

While this is harmless and normally hidden from user, on some servlet configurations this leads to double authentication of page (first is for index.html in apidocs and second goes to "catch all" servlet filter.

How to reproduce:

• load apidocs explorer
• press F12 (to open developer tools in browser)
• refresh page

HTTP404 http://localhost:8181/css/opendaylight.css

Problematic code seems to be restconf/sal-rest-docgen/src/main/resources/explorer/index.html:11

<link rel="stylesheet" type="text/css" href="/css/opendaylight.css">

Leading slash in front of "css" should be removed I guess.

This bug is present in nitrogen, oxygen and master.