Support for directive :toc:, to embed a page table of contents

(@kou suggested this feature request.)

An embedded directive :toc: is replaced by the table of contents (TOC) derived from the down-page headers:

• If the headers are well-formed (see below), the TOC is structured as nested lists; if not, the TOC is a single flat list.
• Each TOC entry is a link to its corresponding header; the text for the link is the text from the header.
• Headers preceding the :toc: directive are not included in the TOC.

The headers are well-formed if:

• The first-found header is as high-level as any headers that follow it. E.g., if the first-found header is level 2, there are no following level-1 headers (but there may be additional level-2 headers).
• There are no level "jumps". E.g., a level-2 header followed by a level-4 header.

The issue names no files or tests. Start by locating RDoc's header parsing and HTML rendering entry points, then trace how page content is converted to output. Done means :toc: creates links for later headers, uses nested lists for well-formed levels, and falls back to a flat list otherwise.