Improve AutoLink Setup documentation

[DHaussermann]

There are 2 main problems with the setup info in the AutoLink read-me.

1. We make no mention of the sysadmin using the UI slash commands

• Name, Pattern and Template can be saved to the config
• Other options such as scope, wordmatch and process bot post can all be set

2. The examples we give for common tools like Jira and GitHub don't work as documented and require tweaking

• Examples that include (?P<id>\\d+) can't be copied directly and used for UI config because there is an extra \ in the string is only used to escape it if you copy directly to the config.
• Examples that include URLs to GitHub and Jira don't work. The URLs don't evaluate as a match the way they are written. ex https://github\\.com/ should look like https:\/\/github.com\/)