Fix trailing slashes in the new docs

[mtrunkat]

All landing pages have trailing slash, for example /api/client/js/. I think that it's fixable by replacing this rewrite:

rewrite ^/api/client/js$ /api/client/js/ redirect;

with

rewrite ^/api/client/js/$ /api/client/js redirect;
rewrite ^/api/client/js$ /api/client/js/index.html;

The user gets then redirected from trailing slash to normal, and that gets internally rewritten to index.html which should work. But I am not 100% sure that there are no downsides :) and it's low prio for now.

CC @B4nan

[B4nan]

We should prioritize this, as it also has other side effects like this:

#595