introduction-protocol.md: fix section structure, linkify #NTOR-WITH-EXTRA-DATA
Based on archaeological excavation of rend-spec-v3.txt
it appears that these
two sections are intended to be peers in the table of contents:
3.3.1. Introduction handshake encryption requirements [INTRO-HANDSHAKE-REQS]
3.3.2. Example encryption handshake: ntor with extra data [NTOR-WITH-EXTRA-DATA]
Unfortunately only 3.3.1
had its section heading markdownified, which causes
renderings to merge them into one big section.
This commit corrects that by adding a markdown heading prefix ###
to "Example
encryption handshake: ntor with extra data". It also includes a link target
{#NTOR-WITH-EXTRA-DATA}.