Update links in README.md

[colbya]

Describe the bug
The links in the README.md point to no longer available Zendesk documentation. Update links to current docs.

Additional context
The README links are labeled to show the status of implementing all the web endpoints. The list does not reflect all the currently available endpoints and may contain some that are no longer available.

I did not see current documentation for these items at zedesk.com (neither were checked as implemented):

• Restrictions and Responsibilities
• Autocompletion

Does Forums still exist?

• Forums ✓
• Forum Subscriptions
• Categories

This link was not removed in the PR but might be something to consider removing:

• Users list

PR to be submitted updating the existing links.