Expand list of sections yet to be written. Arrange sections both written
and unwritten into a two-level structure, with a bit of consequential
reordering.
Added sections on the technic-specific kinds of item processing, and on
generic metal mechanics, and the specific trickery around iron (merging
in notes_on_iron).
Vanessa finds "altitude" to carry too much of an aviation connotation.
Zefram finds "elevation" to carry insufficient connotation of being
relative to sea level, so also adds a clarifying note. Beware of changing
this terminology without negotiation, as it aroused surprisingly strong
opinions.
