Add health warning about running Docker on OSX

Added documentation on Michael McAndrews docker container.

Tim and I added these instructions to amp (and improved them) so now
it's better to remove them here.

Just spotted a typo in my previous change.  That 'v' should have been an 'h'.

The command `which civibuild` returns nothing even if the path is set correctly.

As noted earlier in the doc: "We strongly recommend using Apache as your
webserver because support for nginx is limited." So...  it doesn't make much
sense to give broken/incomplete instructions for something we say you
shouldn't do.  :)

Add in help message about troubleshooting where you cannot login to buildkit build because rewrite mod isn't enabled

Clarify Nodejs instruction line.

Reading for content; editing for consistency.

Edit to paragraph format rather than short lines.

Make the warning about running Ubuntu installation as root slightly more dire.

- General formatting improvements.
- Change  "Ubuntu 12.04 or 14.04" to "Ubuntu 12.04 or later" because the
  buildkit install script is clearly designed to work for later versions
  of Ubuntu.
- Relocate troubleshoot steps to after configuration steps (so the
  reader doesn't stop reading when they see "troubleshooting".
- Higlight post-installation steps.
- New instructions for manually ading webserver settings after running
  `amp config`.
- More troubleshooting help.

This was in the general "buildkit" page, and I think it fits better in
the specific "civibuild" page.

This is a direct copy-paste from one section of the guide to another,
without any edits. When someone is installing buildkit, these steps are
easy to miss if they're located in the civibuild file.

This way MkDocs will be able to report errors for broken links

Done with:

perl -0777 -i -pe \
's@(?<!!)\[([^\]]*)\]\((?!https?://)(?!mailto:)(/(?:(?:(?:/(?![\)#]))|[^/)#])(?!.md))+)/?(#[^)]*)?\)@[$1]($2.md$3)@gm' \
`find . -name '*.md'`

Not adding internal redirects because these pages are so new.

Based on discussion here:
https://github.com/civicrm/civicrm-dev-docs/pull/189