Fix broken links on the Getting Started page

[JeffJLi]

The following links on https://attack.mitre.org/resources/getting-started/ are broken:

• The second link under "Adversary Emulation and Red Teaming" for Do-It-Yourself ATT&CK Evaluations should be updated to http://www.sans.org/cyber-security-summit/archives/download/22695.

• The last link under "Adversary Emulation and Red Teaming" for the ATT&CK Evaluations Adversary Emulation Summary should be updated to https://attackevals.mitre-engenuity.org/adversary-emulation.html.

These issues were initially posted on the ATT&CK Evaluations Github repo here and here.