In the split-HTML version of the tutorial, name sections instead of numbering them

[nyh]

We have in doc/ the scripts md2html and htmlsplit.py that convert tutorial.md into HTML pages that can be seen in https://docs.seastar.io/master/tutorial.html (single HTML page for the entire tutorial) and https://docs.seastar.io/master/split/ (multi-page version, one page per section).

In the single-page version, a link to Section 5 (Coroutines) is https://docs.seastar.io/master/tutorial.html#coroutines - this is nice, because this link remains relevant if a new section is ever inserted before this section and sections get renumbered.

On the other hand, in the multi-page version of this link is https://docs.seastar.io/master/split/5.html#coroutines - note the "5.html" in the URL - this link will break if we ever add or delete or move a section.

The request (made by @avikivity) is to have htmlsplit.py name the separate HTML files by the section's name instead of number.

As we do for anchors, we should replaces spaces in the file name by hyphens (which according to Google's guidelines, is preferred over underscores).

CC @avikivity @tchaikov