Add 'Customizing NetNewsWire' page to v6.1

[jonjahr]

This PR adds a 'Customizing NetNewsWire' page with instructions to hide the badge from the dock icon. Most of the words I copy-pasted from the v5.0 version of this page, except for the 1-liner that now accurately instructs the reader to go to System Preferences.

It also adds a link to the new 'Customizing' page to the main TOC (I added it to the end of the "Get more help" section).

Earlier today I was trying to hide the badge icon. I did a web search and clicked one of the top results, which unwittingly turned out to be the /customizing page from the v5.0 help site. It said to use a checkbox in the application's preferences (which had since been removed in v6.0b3). It was enough to confuse me for a bit. Maybe this page will eventually get indexed above the old v5.0 version and help the next person!

If you want any changes or if I can do anything else, let me know.