Search results have incorrect link if site is served on a subdomain via `BASE_URL`

[choldgraf]

If a MyST site is deployed with a subdomain (e.g. foo.org/mybook/), then search result links are broken because the subdomain folder isn't added to the links.

For example, see the 2i2c CZI report docs.

• The book is served at https://2i2c.org/report-czi-2021/
• The search results look like https://2i2c.org/year1#managed-hub-service-revenue

AKA, they are missing the report-czi-2021/, which is defined in the BASE_URL environment variable.

Here's the documentation I followed to set up the BASE_URL parameter on github actions

[rowanc1]

Should be fixed with:

• https://github.com/jupyter-book/myst-theme/pull/475