Update docs-build.md

[manez]

Purpose / why

Adds a link to a video version of some of these instructions. This just covers the build and deploy bits. It would probably be helpful to have a video covering the installation section as well, but I don't currently have access to a fresh machine to re-install and record. If anyone out there wants to screen record themselves installing mkdocs following our instructions, I'd be happy to turn it into another Quick Lesson.

What changes were made?

New admonition pointing out to the video link.

Verification

Get meta and follow the instructions on this page to build the new version and make sure the admonition looks right 😄

Interested Parties

@Islandora/documentation

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Checklist

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?
• Does this PR update the last updated on date on the documentation page?

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?

[rosiel]

Hi @manez can you update the "last updated" date? (it's in the checklist)

[manez]

Of course! I keep missing that one...