fix: use Equinix red for top nav

[ctreatma]

We already have custom CSS to override the nav bar color, but it doesn't take effect for most of our sites (it's unclear why the color shows up on the Terraform on Equinix workshop pages; the configuration looks identical).

Note that the custom color scheme had to be specified twice, once for light mode and once for dark mode.

Fixes #10

[ocobles]

custom option for primary color was added in mkdocs-material 9.1.8, and indigo was set as default (there was no default option before) and the custom-color documentation updated in 9.1.9.

• https://github.com/squidfunk/mkdocs-material/compare/9.1.7...9.1.9#diff-ffad206157b23683953799af4d3aa11e78adbe27654dc034c049ee877f871c4fR940
• https://github.com/squidfunk/mkdocs-material/compare/9.1.7...9.1.9#diff-ffad206157b23683953799af4d3aa11e78adbe27654dc034c049ee877f871c4fR940

Since our CI uses the latest mkdocs version we must update all existing workshops with this change so that they won't lose the red color in new releases