Per-page customization of website theme options

[choldgraf]

Currently, theme options are set at the project level, meaning that you define it once in myst.yml and it applies to the entire site.

However, there are some cases where users want to apply options at a page level. It would be useful if we had the same project -> page inheritance for site options.

Example: Splash pages vs. documentation

Consider a site that has three top-level sections they wish to put in the navbar, one of which is docs/. Their Table of Contents defines the structure of docs/, and they only want it to show up when users are in docs/*.

To accomplish this, we could use page-level theme options to disable the left and right sidebar only for the non-docs pages.