Move keydefs to a separate file

Moving the keydefs to a separate file so they can be included in submap
builds for faster build times, and also for ease of maintenance.
Refactoring installation into task-oriented topics

- Refactored install for each platform into a single topic so reader has
all they need in one place and does not have to hunt for relevant
information among material describing other platforms.

- Refactored each presentation topic into its own DITA map. This
involved repeating some near-identical material in multiple maps. Can
probably factor out some of this with conrefs, but can't find a way to
substitute different text when conref used in different maps. All
references seem to be global.

- Used chunk attribute and toc="no" in maps to reduce complexity of TOC.

- Factored out several pieces of repeated material.

- Made many edits for language and clarity. More of this still to do.

- Still need to figure out what to do with topics on Java settings and
performance issues, which are more troubleshooting or configuration
than install topics.