Documentation for intro sections inconsistent with parser

[NickHu]

The documentation says that Markdown can be separated into an intro section preceding the rest of the content with

<!-- more -->

However, the parser actually only recognises <!--more--> (no spaces): https://github.com/styx-static/styx/blob/4a1489bb1b358647285006d6a32291ce1a54fa5c/src/tools/markdown-parser.py#L103