Update sbt-microsites

[travisbrown]

I started migrating from Tut to mdoc but it doesn't feel practical:

• We rely very heavily on redefinition. I started putting things in wrapper objects or adding suffixes but when you have half a dozen iterations of def combineAll or whatever this really hurts readability (in some cases :reset is an option but in most it's not).
• We have one reasonable use of nofail.

There's probably other stuff but that's where I gave up.

I do think we should update sbt-microsites sooner rather than later, though, so I've made the bump and specifically set WithTut and the old theme. I built it locally and everything looks fine.

I also updated Jekyll in the Travis CI config, because I did a similar update for Circe this morning and being on an old version of Jekyll caused problems in a non-obvious way that was a huge pain to debug.

[codecov-io]

Codecov Report

Merging #3218 into master will decrease coverage by 0.33%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3218      +/-   ##
==========================================
- Coverage   93.38%   93.05%   -0.34%     
==========================================
  Files         372      376       +4     
  Lines        7224     7412     +188     
  Branches      179      192      +13     
==========================================
+ Hits         6746     6897     +151     
- Misses        478      515      +37

Flag	Coverage Δ
#scala_version_212	93.38% <ø> (ø)	⬆️
#scala_version_213	92.82% <ø> (?)

Impacted Files	Coverage Δ
...e/src/main/scala-2.13+/cats/data/ZipLazyList.scala	77.77% <0%> (ø)
...src/main/scala-2.13+/cats/instances/lazyList.scala	100% <0%> (ø)
....13+/cats/kernel/instances/LazyListInstances.scala	100% <0%> (ø)
.../main/scala-2.13+/cats/data/NonEmptyLazyList.scala	62.5% <0%> (ø)
...in/scala/cats/data/AbstractNonEmptyInstances.scala	100% <0%> (+3.44%)	⬆️

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