Add guides for importing existing databases and uploads to the local environments

The getting started guide assumes people are starting from scratch and doesn't help much with regards to importing existing databases.

We should add a guide for this and link to it from the end of the getting started section as well.

**Note:** Blocked on humanmade/altis-dev-tools#206 which will change which commands to document and run.

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## Acceptance criteria:

- [ ] Go through the process of downloading a db backup and uploads from Altis Dashboard taking screenshots as you go of:
- The Imp / Exp tab and screen
- The Popup with both Database & Uploads selected
- [ ] Unzipping the output
- [ ] Copying uploads to `content/uploads`
- [ ] Copying the db zip file to your project root or path under the project root

### For Local Chassis:

- [ ] Document steps needed to import the database:
- Import: `composer chassis exec -- wp db import `
- Search & replace with example

### For Local Server:

- [ ] Document steps to import db:
- Import: `composer server cli -- db import `
- Search & replace with example, note must be to a subdomain of `altis.dev`
- [ ] Document synchronising uploads to S3 container:
- `composer server cli -- s3-uploads upload-directory content/uploads uploads`
- Explainer eg. S3 is more like a database than standard file storage
- Note that anything uploaded to the site will automatically be synced to the local uploads directory

**Side note** that for Altis v4 image import will be `composer server import-uploads`, we can update that once this initial guide has been merged and backported.

### Troubleshooting section:

- [ ] Flushing the cache