[DOCS] Update doc/security/index.rst to link to the SECURITY.md file in the master branch

[ScriptAutomate]

Description

Right now, the SECURITY.md file is representative of the latest updates around contact information and gpg public key for security reporting.

Though, in the Salt docs, we have a duplicate file, doc/security/index.rst, that can be out-of-sync with SECURITY.md.

This .rst file is really a duplicate of the information in SECURITY.md.

Suggested Fix

The doc/security/index.rst file should just link out to the SECURITY.md file within the repo root of the master branch.

Type of documentation

• Salt documentation

Location or format of documentation

Published docs (coming from doc/security/index.rst of the latest release tag for that particular version, except master reflects the master branch):

• 3006 (currently up-to-date): https://docs.saltproject.io/en/3006/security/
• 3007, or latest (currently out-of-date, because it is dependent on a new point release to update): https://docs.saltproject.io/en/latest/security/
• master (currently up-to-date): https://docs.saltproject.io/en/master/security/

The best route for the solution would be to submit a PR against the 3006.x branch with an update to doc/security/index.rst, which will then be included in subsequent merge-forwards.