[doc] Restrict keyword search to individual blocks

### Description

There are broadly speaking two types of search that can be done in the documentation:

1) A global search across the whole book
2) Local search (i.e. the browser's find) within a specific page

Unfortunately there doesn't seem to be a way to search for a particular word within a chapter or sub-chapter. Since IP blocks are broken up into multiple sub-chapters it is difficult to search for a particular word (e.g. `ACK`) across all the documentation for a particular block.

@rivos-eblot @marnovandermaas @HU90m @hcallahan-lowrisc

Start by reviewing the documentation structure and the current global and local search behavior described in the issue. Determine how IP blocks and their sub-chapters are represented, then define and verify a way to restrict keyword searches to one block; no files or tests are identified in the issue.

Written by the indexing model from the issue text.