Anchors broken for home page

From some applications (e.g. Gmail and Slack OSX Desktop app), anchors on the home page are broken.

This is because they turn two consecutive hashes (`##`) into the second one url-escaped (`#%23`).

E.g., if you paste this into slack app or send in gmail, then click it:

`https://mysite.com/##header`

You get sent to this page (which obviously 404s):

`https://mysite.com/#%23header`

This only happens on the root page (which is the only one with consecutive hashes in the URL), so one solution would be to use `mysite/#/` (with a trailing slash) as the root, instead of just `mysite/#`. However, this seems to work on real sites (served via nginx), but not when served locally via `mkdocs serve`.