Aside from updating the doc structure, this commit
also rephrases step 4 to make it read better. The
list of supported events in step 5 is also removed.
Part of #29592.
markdown-include is GPL licensed.
Also, rewrite it as a block processor, so that it works correctly
inside indented blocks.
Signed-off-by: Anders Kaseorg <anders@zulip.com>