Refactors the handler for the AsmDocs API to reduce code duplication.
**Breaking changes proposed**
- API will now return 404. This used to return 200 with "Unknown opcode" which is what led to issue #3117. Following cases has been changed:
1. archiecture was not found
2. instruction opcode was not found
- API will now return 406 not acceptable when the accepted content type is not text or json
- There is no longer a `result` field on a 200 request, instead the assembly documentation object is returned.
- There is no longer a `found` field on any request, the HTTP codes indicate whether it was found or not (200 means found)
**New features**
- The frontend will only request the opcode for an instruction set at most once, caching every single outgoing request.
- The "There was an error fetching the documentation for this opcode" has been adjusted to contain what went wrong.
Relevant tests have been added and updated.
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