Remove "Hacking on Islandora 8" for #1794

[manez]

Purpose / why

Remove obsolete documentation page Hacking on Islandora 8 to address #1794

What changes were made?

Removed a page and edited mkdocs.yml to take it out of the menu. Since it was in a a nested list with only one other item, I removed the parent and bumped up the other entry.

Verification

This involves the menu, so probably worth a test docs build.

Interested Parties

@dannylamb or @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?