Related to #3135
Update REST APIs documentation to include default and max values for `page_size` flag
* **Client Code Changes**
- Update `getStoreMessagesV3` function in `waku/waku_api/rest/store/client.nim` to set the default value of `page_size` to 20.
* **Handler Code Changes**
- Update `installStoreApiHandlers` procedure in `waku/waku_api/rest/store/handlers.nim` to enforce the default value of `page_size` to 20.
- Enforce the max value of `page_size` to 100 in `installStoreApiHandlers` procedure.
* **Documentation Changes**
- Add a note in `docs/api/rest-api.md` mentioning the default value of `page_size` is 20 and the max value is 100.
- Add a note in `docs/operators/how-to/configure-rest-api.md` mentioning the default value of `page_size` is 20 and the max value is 100.
* Rename waku_api to rest_api and underlying rest to endpoint for clearity
* Rename node/api to node/kernel_api to suggest that it is an internal accessor to node interface + make everything compile after renaming
* make waku api a top level import
* fix use of relative path imports and use default to root rather in case of waku and tools modules
