Reorganize finite sample material, but leave in intervals
Restore confidence interval material to estimation document

This commit restores the old confidence interval material that was
accidentally erased earlier.
Edit interval estimation text

This commit rewrites the existing interval estimation material in
sentences and paragraphs.  It also adds a brief discussion of
optimality.
Edit optimality material in estimation document

This commit rewrites the optimality material to use paragraphs
and sentences instead of a terse outline.
Rewrite and move some linear regression material

This commit rewrites some of the finite-sample linear regression material
to use words and paragraphs, etc.  It also moves the algebraic and statistical
(finite-sample) properties of linear regression back with the rest of the
finite-sample material than they do on their own.