Update install-kubectl.md to use tabs for the different Operating Systems (#26682)
* top-level tags and change to included files
* revert content to main windows page
* include gcloud and update toc
* add leading blank line
* add text to avoid YAML error from Netlify
* remove YAML delimiter error
* removed redundant weight key
* add front matter and toc shortcode
* test to see if toc is fixed in included page
* update macos toc to show difference
* use static toc instead of shortcode
* uniqueness for gcloud install headers
* Move to model suggested in https://github.com/kubernetes/website/pull/26682#discussion_r585091797
* correct link to included/install-kubectl-gcloud.md
* correction of links to per-OS pages
* try a different way to use path to per-OS links
* correction of brainfart
* remove .md from link destination for per-OS pages
* modify how auto-competion steps are displayed
* remove redundant file
* correction of include - change % to <
* remove closing tag as not needed
* try and hide this directory from the ToC on the left
* remove erroneous "include="
* addressing feedback: https://github.com/kubernetes/website/pull/26682#discussion_r586325123
* addressing feedback: https://github.com/kubernetes/website/pull/26682#discussion_r586326346
* addressing feedback: https://github.com/kubernetes/website/pull/26682#discussion_r586326604
* addressing feedback: https://github.com/kubernetes/website/pull/26682#discussion_r586327212
* https://github.com/kubernetes/website/pull/26682#discussion_r586327856
* consistent verb usage as per feedback: https://github.com/kubernetes/website/pull/26682#discussion_r586328497
* update redirects as existing page is being deleted
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