[Snyk] Security upgrade github-pages from 230 to 231

[jwagantall]

Snyk has created this PR to fix 1 vulnerabilities in the rubygems dependencies of this project.

Snyk changed the following file(s):

• docs/Gemfile
• docs/Gemfile.lock

Vulnerabilities that will be fixed with an upgrade:

	Issue	Score
	Improper Restriction of Recursive Entity References in DTDs ('XML Entity Expansion') SNYK-RUBY-REXML-12878608	631

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[jt-nti]

This PR is missing a DCO signoff. I don't think the vulnerability is a concern for the static site but I'll investigate when I get a chance.

[jt-nti]

I've opened #264 to update github-pages