[!151] Document pyramid lfe

Merge branch 'document-pyramid-lfe' into 'master'

ref:core/dune-localfunctions> These elements are nontrivial, and need some documentation.

Fixes #3

See merge request core/dune-localfunctions!151

Closes #3 (closed)

(cherry picked from commit c95bc4bf)

a4a35d2b Document the Lagrange finite elements for pyramids 4d665d92 Fix two typos in documentation