Problems with trailing slashes on the new docs path

[rogermparent]

For some reason, at some point after #1257, trailing slashes appear in the hrefs of links pointing to the new homepage at "/doc", but inconsistently as the effect is removed upon navigation.

This issue is to investigate and fix that.

[rogermparent]

From my investigation so far, it seems to be related to the sidebar which hasn't actually been completely decoupled from link logic. The normalized sidebar has the doc page as /doc/ despite everything else, even the internal Gatsby node, having no trailing slash.
This issue alone shouldn't be too hard to patch up quickly, but it indicates we're in need of a bigger fix to actually separate this sidebar or re-integrate it in a better way. That said, that fix should be another issue and this one should just fix the problem in front of us now.