Update staging site instructions

[joshdentremont]

Purpose / why

New instructions for using docker-compose.override.yml to store a staging site URL outside of the git repo

What changes were made?

Updated instructions for staging site URL and added link to IPAllowList middleware for IP restrictions

Verification

Try the example docker-compose.override.yml to see if it can change the URL that you have specified in your .env.

Interested Parties

• @Islandora/documentation
• @Islandora/committers

Checklist

Pull-request Reviewer

Pull-request reviewer should ensure the following:

• Does this PR link to related issues?
• Does the proposed documentation align with the Islandora Documentation Style Guide?
• Are the changes accurate, useful, free of typos, etc?

Person Merging

The person merging should ensure the following:

• Does mkdocs still build successfully? (This is indicated by TravisCI passing. To test locally, and see warnings, see How To Build Documentation.)
• If pages are renamed or removed, have all internal links to those pages been fixed?
• If pages are added, have they been linked to or placed in the menu?
• Did the PR receive at least one approval from a committer, and all issues raised have been addressed?

[super2014001]

您好！您的邮件我已收到！谢谢！