keep page TOC always visible on hledger.org

[simonmichael]

As noted on #1895, currently on hledger.org the page table of contents scrolls out of view (unlike the site table of contents). It might be useful to keep it visible, especially on long pages like the hledger manual, eg in the style of mkdocs-material linked there. Perhaps borrow CSS/JS from there, or maybe there's a plugin for mdbook which we can use. We'd like someone to try this and explore the issues of screen space usage, browser compatibility, etc. noted there.