Updates related to search function

[lhelms2]

1. See if there is a way for users to search sibling subprojects from within a subproject (i.e. return search results from ICRN when searching in the Delta docs). Currently docs are set up to only allow subproject search results when searching from the documentation hub (parent project).

2. In the meantime, integrate pointing users to search from the documentation hub if they aren't finding answers when searching in their subproject. The 'exploring solutions' section of 'getting help' could be a good place.

These were identified/discussed during the user documentation meeting on 9/11.

[lhelms2]

1. Per the RTD docs, they don't yet support sharing search results between sibling subprojects. https://docs.readthedocs.io/en/stable/subprojects.html#search

[lhelms2]

2. Added info about searching from the hub to the 'explore solutions' section in Delta and DeltaAI 'getting help'