Add "Companies using Splinter" page to docs

It would be nice to know what companies are using splinter for two reasons: marketing and the feeling of "if company X is using splinter I should give it a chance."

Start by locating the existing documentation structure and conventions for adding pages. Clarify which companies and supporting details should be included, then add the page in the appropriate documentation location and verify that it appears in the docs navigation or build.

Written by the indexing model from the issue text.