Search page tidying up

Tidy up of the search page (while reading the re-indexing section). Main changes:
- unify headings to comply the documentation policies 
- use `file` and `envvar` markup and `$` as the prompt separator 
- add percent indexed SQL command (see https://www.openmicroscopy.org/community/viewtopic.php?f=5&t=7502&p=14120&sid=2d59ace548a73d2741172b7c22cf7352#p13893) /cc @kennethgillen
- convert large table (> 78 characters) into `list-table` for clarity 
- wrap lines at 78 characters
Move this paragraph to the developers docs? (source/API links would be nice BTW :))