Move style guide into subdir for future splitting

What does this MR do?

Moves the style guide into its own directory as an index.md file, and chops a few subheadings out into a markdown.md file. The style guide is (IMO) horrifyingly long and unwieldy, and I think we need to split it somehow. I'm not certain what I've got is the right answer, but I want to push us to start thinking about AN answer.

I don't think we need to answer "how do we tidy up the entire style guide?" all at once, but if we could start somewhere, I think the TW team would benefit. cc @gl-docsteam because they will have strong opinions here.

Screenshots (strongly suggested)

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