"Security Doucmentation" --> "Security Documentation"

Alter headings and add in a Javascript intro paragraph, Attempt to mae it clear Angular and others are not part of the AJAX interface

Close off issue 120 by documenting cv-nodejs and node-civicrm js bindings that can be used to interact with CiviCRM

Minor corrections.

Adds documentation for https://github.com/civicrm/civicrm-core/pull/10984

With only one example, a reader might misinterpret the example as saying
that `Search Preferences` has been renamed `domain`.  Including two examples
should more clearly indicate that *all* group-names are now flattened to
just `domain`.

This section documents two distinct changes, but they had different
formatting -- which made it seem that (perhaps) the one code-block was
describing both changes.

This change uses similar formatting for both examples and changes the
indentation/alignment to indicate that they're distinct.

Issue 92 Rename API Paramaters to API Options and also port in Custom Data Documentation to help with Issue 92

- Conform to standards
- Fix broken links

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'`