Update Documenter to 0.26

[omus]

May fix #64. I noticed in the Documenter CI job that GITHUB_TOKEN wasn't listed which may be because we are using an old version of Documenter.jl:

┌ Info: Deployment criteria:
│ - ✔ ENV["TRAVIS_REPO_SLUG"]="" occurs in repo="github.com/beacon-biosignals/K8sClusterManagers.jl.git"
│ - ✘ ENV["TRAVIS_PULL_REQUEST"]="" is "false"
│ - ✔ ENV["TRAVIS_TAG"]="" is (i) empty or (ii) a valid VersionNumber
│ - ✘ ENV["TRAVIS_BRANCH"]="" matches devbranch="main" (if tag is empty)
│ - ✔ ENV["DOCUMENTER_KEY"] exists
│ - ✔ ENV["TRAVIS_EVENT_TYPE"]="" is not "cron"
└ Deploying: ✘

[omus]

Looks much better:

 ┌ Info: Deployment criteria for deploying preview build from GitHub Actions:
│ - ✔ ENV["GITHUB_REPOSITORY"]="beacon-biosignals/K8sClusterManagers.jl" occurs in repo="github.com/beacon-biosignals/K8sClusterManagers.jl.git"
│ - ✔ ENV["GITHUB_REF"] corresponds to a PR number
│ - ✘ `push_preview` keyword argument to deploydocs is `true`
│ - ✔ ENV["GITHUB_ACTOR"] exists and is non-empty
│ - ✔ ENV["DOCUMENTER_KEY"] exists and is non-empty
└ Deploying: ✘

[codecov[bot]]

Codecov Report

Merging #67 (3691706) into main (270a59c) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #67   +/-   ##
=======================================
  Coverage   67.63%   67.63%           
=======================================
  Files           4        4           
  Lines         173      173           
=======================================
  Hits          117      117           
  Misses         56       56           

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