storybook: Addon-docs: can't set link with absolute path.

Describe the bug When I try add link to some external page, either with markdown or anchor element, this URL automatically added to path parameter instead. On other hand, if I add URL without leading slash, it works as expected - as relative link.

To Reproduce Steps to reproduce the behavior:

1. Create new or open existing .mdx doc.
2. Add link to some external page, e.g. /example/:

   [absolute example](/example/)
   [relative example](example/)
   

3. Go to this page in started or builded storybook
4. First link points to ?path=/example/, and second points to %storybook_path%/example

Expected behavior

1. Both links should work similarly. Either point to pages inside storybook, or to external pages
2. There should be clear way to add links to other parts within same domain.

Screenshots

Code snippets

[absolute example](/example/)
<a href="/example">absolute example</a>
[relative example](example/)
<a href="example">relative example</a>

System:

$ npx -p @storybook/cli@next sb info

Environment Info:

  System:
    OS: Linux 4.19 Ubuntu 18.04.3 LTS (Bionic Beaver)
    CPU: (8) x64 Intel(R) Core(TM) i5-8250U CPU @ 1.60GHz
  Binaries:
    Node: 10.16.3 - ~/.nodenv/versions/10/bin/node
    Yarn: 1.19.1 - /usr/bin/yarn
    npm: 6.9.0 - ~/.nodenv/versions/10/bin/npm
  npmPackages:
    @storybook/addon-a11y: ^5.3.0-beta.10 => 5.3.0-beta.10
    @storybook/addon-docs: ^5.3.0-beta.10 => 5.3.0-beta.10
    @storybook/preset-typescript: ^1.1.0 => 1.1.0
    @storybook/react: ^5.3.0-beta.10 => 5.3.0-beta.10