Initial Auto DevOps concept content update

What does this MR do?

This MR is an initial effort to improve the Auto DevOps intro section with information that better describes its benefit to users. It's closely based on the information on the Marketing page of the same topic (at https://about.gitlab.com/stages-devops-lifecycle/auto-devops/), but also incorporates some of the existing information already on the doc page. It also removes the unneeded Overview section header.

More work is to be done, but this hopefully provides an initial push on the topic.

Related to technical-writing#395 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team