Change "Installation" to "Install" in page titles

We should make our get started sections have all verb titles. Noun forms are naturally more ambiguous, and everywhere in the docs except here we use imperative verbs to introduce procedures. The reasoning relates to the removal of Gerunds described in #818.

- [ ] Update file paths and Page titles
- [ ] Fix all instances of internal links to new path
- [ ] Program redirects in `gatsby-node.js`
- [ ] Test that these redirects work in staging, and that they don't introduce any broken links (much easier now that we have a linkchecker).

Start by locating the get started page titles and paths that use “Installation,” then review gatsby-node.js for the redirect entry points. Update the paths, titles, and internal links, and use the staging site and linkchecker to confirm redirects work without broken links.

Written by the indexing model from the issue text.