Some minor documentation enhancements.
Merge pull request #1112 from pelson/docs_tidy

DOC: Minor documentation enhancements.
making in-place addition modification suggested in issue #1089
Merge pull request #1116 from rebeccaroisin/inplace_addition

Speed-up through in-place addition (closes gh-1089)
 Website font readability improvements.

 Increase font size by 1%
 Increase character spacing
 Increase font size in code snippets to website standard
Merge pull request #1121 from Cadair/doc_theme_tweaks

 Website font readability improvements.