Fix nav.html include path in index.html

[stupidloud]

Fix a broken include path in src/docs/json/index.html by changing {{include "/includes/docs/nav.html"}} to {{include "/old/includes/docs/nav.html"}}, so the JSON docs page loads the correct nav partial.

[mholt]

What? Why? What is broken?

[stupidloud]

Hi @mholt, sorry for the terse PR description — let me explain what I ran into.

On the current master, visiting /docs/json/ fails to render the docs nav because src/docs/json/index.html includes /includes/docs/nav.html, but that file doesn't exist in the repo. src/includes/docs/ only contains details.html, hovercard.html, and renderbox.html. The actual nav.html lives at src/old/includes/docs/nav.html.

The sibling page src/docs/modules/index.html already includes the nav from /old/includes/docs/nav.html, so this PR just brings json/index.html in line with it. Happy to take a different approach if you'd prefer the file moved/symlinked instead — just let me know.