Update Cover art options description

- typo fixes
- remove reference to the old plugin (<1.2)
- add a note about 1.3 release group image fallback option
matched -> match
Merge pull request #16 from zas/cover_art_options

Update Cover art options description
Merge remote-tracking branch 'upstream/master' into i18n
Better handling of page's titles

- create a new docs/base.html template
- ensure document title is coherent