Remove old content and place redirects where the content is adequately
replaced by the new walkthrough. Leaves some content which does not
overlap the walkthrough. Moved reference information about package
contents.
Test-bot: skip
1. Copy help.keyman.com/developer/17.0/* into /developer/docs/help
2. Update references to 18.0 where appropriate
3. Copy all referenced images from /cdn/dev into
/developer/docs/help/images and update links in documentation
Next commit: add deployment to help-keyman-com.sh. Note: simplest way to
test links at this point is to use the help.keyman.com auto deploy and
wait for the PR on help.keyman.com to be validated; we could look at an
internal link validation tool that verifies markdown links?
Relates-to: #12347
