Trim Fira fonts that were unused.

This trims out 5.0KB uncompressed and about 0.5KB when compressed.
If the removed font (700 weight) is actually used somewhere then the
savings are larger, but quality will suffer. The 600 weight font should
be used instead if anyone finds a 700 weight Fira Sans.
Clarify that the manual is covered by a different licence.

This came up on IRC: a user thought that this page also covered the manual,
which is a reasonable assumption if you don't know that the manual and Web site
are different subprojects.