jekyll: Do not use strict mode

[EricCousineau-TRI]

Use contains for explicit nested config check for one missing var
See: Shopify/liquid#1034 (comment)

From failure: https://drake-cdash.csail.mit.edu/test/129053560

  Liquid Exception: Liquid error ({runfiles}/drake/doc/_includes/header.html line 15): undefined variable subfolderitems included in /_layouts/default.html
jekyll 3.8.6 | Error:  Liquid error ({runfiles}/drake/doc/_includes/header.html line 15): undefined variable subfolderitems included 
/usr/lib/ruby/vendor_ruby/liquid/variable_lookup.rb:62:in `block in evaluate': Liquid error ({runfiles}/drake/doc/_includes/header.html line 15): undefined variable subfolderitems included  (Liquid::UndefinedVariable)

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This change is 

[jamiesnape]

Reviewed 1 of 1 files at r1.
Reviewable status: needs platform reviewer assigned, needs at least two assigned reviewers

[EricCousineau-TRI]

@drake-jenkins-bot linux-focal-unprovisioned-gcc-bazel-experimental-documentation please

[EricCousineau-TRI]

+@jamiesnape (retroactive)
+@jwnimmer-tri for platform, please (unless you want to force-merge Jamie)
+(priority: emergency)

Reviewable status: 1 unresolved discussion, LGTM missing from assignee jwnimmer-tri(platform) (waiting on @EricCousineau-TRI and @jwnimmer-tri)

a discussion (no related file):
Working: Requires success from: https://drake-jenkins.csail.mit.edu/job/linux-focal-unprovisioned-gcc-bazel-experimental-documentation/4/

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[jwnimmer-tri]

platform.

That is all ye know on earth.

Reviewed 1 of 1 files at r1.
Reviewable status: 1 unresolved discussion (waiting on @EricCousineau-TRI)

[jamiesnape]

I guess we let the documentation build finish since it is running.

[jamiesnape]

CI is already noisy enough today that one extra ping is not going to be an issue if someone else merges something.

[EricCousineau-TRI]

That is all ye know on earth.

I only know of Keats through the Hyperion series; maybe I'll learn more through this campaign :P

[EricCousineau-TRI]

@drake-jenkins-bot linux-focal-unprovisioned-gcc-bazel-experimental-documentation please

[EricCousineau-TRI]

Reviewable status: 1 unresolved discussion (waiting on @EricCousineau-TRI, @jamiesnape, and @jwnimmer-tri)

a discussion (no related file):

Previously, EricCousineau-TRI (Eric Cousineau) wrote…

Working: Requires success from: https://drake-jenkins.csail.mit.edu/job/linux-focal-unprovisioned-gcc-bazel-experimental-documentation/4/

K, I believe this patch will fail in CI. Trying with chroot via Singularity containerization:
https://gist.github.com/EricCousineau-TRI/ee8a89e0164dab5b58766bc45947bea9

Give me 5 more min. Then I'll give up.

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[EricCousineau-TRI]

Reviewable status: 1 unresolved discussion (waiting on @EricCousineau-TRI, @jamiesnape, and @jwnimmer-tri)

a discussion (no related file):

Previously, EricCousineau-TRI (Eric Cousineau) wrote…

K, I believe this patch will fail in CI. Trying with chroot via Singularity containerization:
https://gist.github.com/EricCousineau-TRI/ee8a89e0164dab5b58766bc45947bea9

Give me 5 more min. Then I'll give up.

K, was able to repro error here: https://drake-cdash.csail.mit.edu/test/129305583

Fixed it in r3, it now serves the website. I'm calling this done.

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[jwnimmer-tri]

Reviewed 1 of 1 files at r2, 1 of 1 files at r3.
Reviewable status: complete! all discussions resolved, LGTM from assignees jwnimmer-tri(platform),jamiesnape

[EricCousineau-TRI]

Skipping slow builds.