Adding instructions for updating major versions of buildkit

Purpose / why

Updating Isle from 1.x to 2.x is a bit more complicated than just changing the TAG

What changes were made?

• Small tweaks to the existing instructions to indicate that we should stop our containers before upgrading
• Generic instructions for updating Solr and Drupal/PHP after an update
• Specific instructions for updating from buildkit 1.x to 2.x

Verification

Someone who is still running Buildkit 1.x should follow the new instructions and make sure that they work and are thorough enough to upgrade. Ideally, someone with a production site running 1.x should test these instructions against their dev and production sites.

Interested Parties

@DonRichards mentioned in the Tech call that he would like to test this out.

@Islandora/documentation may also be interested in this

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?