Pro 3778 document flow

[BoDonkey]

Summary

Summarize the changes briefly, including which issue/ticket this resolves. If it closes an existing Github issue, include "Closes #[issue number]"

This PR rearranges the document order in the a3 documentation. The documentation would now have an introductory section that addresses what ApostropheCMS is and what tech stack it uses. The next section would walk users through setting up the environment and bringing up the first project. The remainder of the sections would remain the same: essential guide documentation, advanced topics, and reference materials. Closes PRO-3778.

What are the specific steps to test this change?

For example:

1. Run the website and log in as an admin
2. Open a piece manager modal and select several pieces
3. Click the "Archive" button on the top left of the manager and confirm that it should proceed
4. Check that all pieces have been archived properly

1. Build the documentation
2. Check the flow.
3. Check that home page, menu, and sidebar links work correctly.

What kind of change does this PR introduce?

(Check at least one)

• [ ] Bug fix
• [ ] New feature
• [ ] Refactor
• [X] Documentation
• [ ] Build-related changes
• [ ] Other

Make sure the PR fulfills these requirements:

• [X] It includes a) the existing issue ID being resolved, b) a convincing reason for adding this feature, or c) a clear description of the bug it resolves
• [ ] The changelog is updated
• [ ] Related documentation has been updated
• [ ] Related tests have been updated

If adding a new feature without an already open issue, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

[linear[bot]]

PRO-3778 Restructure A3 documentation for more logical flow.

The introductory section of the a3 documentation jumps from section to section and isn't logical with respect to workflow. The documentation should flow from an introduction (along with looking at the demo), to set-up, to starting a project, and then into the main documentation about how that project can be modified.