Websupport: docroot appears to be ignored

[Mathiasdm]

Describe the bug
I want to make Sphinx documentation available through a docs/ folder using the Sphinx websupport.
However, if I link from one file to another, the generated link always points to the top-level, instead of adding 'docs/' in front (using 'docroot').

To Reproduce
Steps to reproduce the behavior:

1. Use: support = WebSupport(..., docroot='docs')
2. Call: support.build()
3. Using design.rst and design-current.rst, link from design to design-current like: ::doc:`design-current`
4. Run a flask application that calls (1) and (2) and look at the resulting HTML. The link to design-current is not docs/design-current, but /design-current.

Expected behavior
The link should be docs/design-current instead of /design-current.

Your project
n/a

Screenshots
If applicable, add screenshots to help explain your problem.

Environment info

• OS: Linux - CentOS 7
• Python version: 3.6.4
• Sphinx version: 3.5.1
• Sphinx extensions: none
• Extra tools: Browser