[4565] Rewrite/enhance controller-isolation page

[StackScribe]

Implement #4565 .

• Change title. "Controller isolation" is the point but it seems confusing to use this as a title rather than the "distributed builds" terminology that is used everywhere else.
• Moved this page so it will be the first section after index.adoc (after the securing-jenkins.adoc file is removed, with its information moved into other files.
• Rewrote and expanded the information about why this is an important security issue and merged this information with the material that was previously in the index.adoc file. We may have too much redundancy in this material and a couple of superflous points; I need other opinions about this. And it would be good to expand the information about not running agents in the same Docker container as the controller.
• Added definitions of controller, node, agent, and executor. These long definitions might better be placed in the "Managing Nodes" page with a briefer list here but I added them here for now.
• Restructured/expanded information about implementing distributed builds and made it a separate sub-section.
• Rewrote information about the Agent -> Controller Access filter
• Moved the cautionary note about periodically reviewing security settings so it will be after the general introduction to the "Configure Global Security" page that will be in the managing-security.adoc file
• Removed the introductory note in managing-security.adoc file about Jenkins 1.x not having many of these security protections.

@daniel-beck @Wadeck @MarkEWaite

[daniel-beck]

not ready for review

You should be able to mark this PR as draft to make this even clearer.

[StackScribe]

@daniel-beck This is ready for review now. I just saw something I wanted to change so marked it WIP for about an hour while I did that.

[daniel-beck]

Obsolete comment now, I guess, if we're keeping these two pages around? (Why?)

[StackScribe]

I plan to remove the access-control section and replace it by sections about Security Realm /Authentication and another one about Authorization. Just trying to make incremental changes, one smallish PR at a time. Is there a better way for me to go about this? Should you and I have a chat about the changes you want for this chapter? I started out just trying to incorporate some information I learned from you and to make the terminology match what shows on the UI pages but I am happy to do anything you like.

[daniel-beck]

Should you and I have a chat about the changes you want for this chapter?

Yep, I think that's a good idea, see Gitter 😃

[StackScribe]

This content was moved to commit 033a397 so I am closing this PR.