SI-6017 Generate Scaladoc's index links in Scala side

There is no reason to do it in JavaScript.
SI-6017 Scaladoc: Show all letters without dangling links

Use <span> instead of <a href="..."> if there is no page on the letter.
Merge pull request #2050 from kzys/jira-6017-v2

SI-6017 Scaladoc: Fix dangling links on index.html