[ic-design-system]: Migrate the "Custom theme" page to the "Styles" section of the guidance site

[GCHQ-Developer-112]

What component is this related to?

"Custom theme" page / ic-theme

Why do we need it?

Currently, the custom theme page is in the "Install components" section of the website. A few people have had issues trying to find the guidance around theming. This ticket is a suggestion to move the page to somewhere more obvious, maybe styles or patterns or even a component page.

This ticket may require some user research into the information architecture of the guidance site.

[MI6-255]

Consider if we want a development area like other design systems, may be more appropriate