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

[roborourke]

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 <db file path>
  • Search & replace with example

For Local Server:

• Document steps to import db:
  • Import: composer server cli -- db import <db file path>
  • 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

[rmccue]

Related: humanmade/altis-local-server#345

[rmccue]

These steps may have changed since we originally wrote this; additionally, the simplified command we're discussing in humanmade/altis-local-server#345 are likely to change the process here again.