[DOCS]: Add topic describing Salt System Architecture for newbies

[barbaricyawps]

What does this PR do?

This PR is a docs-only change that includes two changes:

• It makes some minor changes to the index.rst file to update the phrasing
• It adds a new topic called Salt System Architecture that describes the core components of Salt (including a fancy image)

What issues does this PR fix or reference?

Fixes:

Merge requirements satisfied?

[NOTICE] Bug fixes or features added to Salt require tests.

• Docs
• [N/A] Changelog - https://docs.saltstack.com/en/master/topics/development/changelog.html
• [N/A] Tests written/updated

Commits signed with GPG?

Yes? I think?

Please review Salt's Contributing Guide for best practices.

See GitHub's page on GPG signing for more information about signing commits with GPG.

[ScriptAutomate]

This looks great to me. The broken tests are unrelated, though.

Can we get final review and sign-off by @dwoz and @garethgreenaway ? And then override the passing tests requirements? I've pulled locally and built, and it looks fine.

git fetch upstream pull/58664/head:pr58664
git checkout pr58664

# Passes nox docs build
nox -e 'docs-html(compress=False, clean=True)'

# Renders great
firefox doc/_build/html/topics/salt_system_architecture.html

It also is tagged for easy reference from other docs in the repo, and appears toward the top in the index / TOC.