use hasdoc() for logo link

[drammock]

closes #912

@datapythonista can you test with this branch? Locally I tested:

• link="https://google.com" (works)
• link="user_guide/install" (works; resolves to http://127.0.0.1:39645/user_guide/install.html)
• link="/" (resolves to http://127.0.0.1:39645/, AKA localhost:port)
• link="/foo" (resolves to http://127.0.0.1:39645/foo)

...but I don't have an easy way to test your setup (sphinx root is not / but rather /docs/, and you want logo to link to /).

[datapythonista]

lgtm, my setup is actually 3. By default the logo link would be to the root page (first if condition, e.g. ./index.html meaning /docs/index.html in my case), and what I need is just /, so all good with this.

Thanks a lot for the PR!