Remove TOC

This PR aims at removing markdown-toc-unlazy, which causes issues with CRA v5: !365 (merged).

This dependency was used to generate the table of contents displayed on the right-hand side of markdown content pages with sub-headings, like "Forms", "Layout Manager", etc.

As discussed, this table of contents was not particularly useful, so I've just removed it. If some content becomes more difficult to find as a result, it can be moved to a sub-page in order to appear in the navigation sidebar.

While I was at it, I tweaked a few things:

• widened the main layout to 75 rem;
• widened the navigation sidebar a little;
• configured the navigation tree to be collapsed by default;
• subsequently improved the UX of the collapsed navigation tree (chevrons for expandable items, collapse an item on click only if it is already active);
• removed "Daiquiri UI" title in the sidebar to save space;
• tweaked font sizes of headings;
• merged the two "Getting started" sub-pages ("Installation" and "Configuration" into a single page).