Raise error in assembly on unknown domain type

Rather than silently falling through in assemble if the domain type is
one we don't know about, produce a useful error message.
Add some text on defining variational problems
Link variational problem section into doc front page
Fix missing links and immersed/embedded confusion
Document init_cell_orientations in immersed manifold section
Add link to boundary condition docs
ToC + wordsmithing
Replace manual links with toctree
Merge pull request #147 from firedrakeproject/better-solving-errors

Don't silently continue when assembling on unknown domain type.
Fix typo
More wordsmithing
More linkifying in documentation page
Final wordsmithing
Merge pull request #136 from firedrakeproject/variational-problems

Add some text to the manual on defining variational problems.
Currently links to some non-extant manual sections.