Fixes GRAPH-610

1. We prepend a markdown-formatted signature to the hover text if SignatureDocumentation field if present
2. If the field is absent, we fall back to old behaviour of assuming that signature documentation is already present in the hover text

SignatureDocumentation was added to SCIP format a while back but it's not yet being utilised by our indexers. One of the first to do so will be scip-java (sourcegraph/scip-java#707) – with the plan for rollout being:

1. Merge this PR and wait for a Sourcegraph release
2. Merge scip-java PR and tag a release
3. (optionally) update the default scip-java image in SG

This change allows handling both new SCIP indexes (those that emit SignatureDocumentation), and the old ones, which have signature prepended to Documentation field.

Test plan

• New tests for the signature and documentation

Changelog

• The backend now respects the SCIP signature_documentation field, and if available, uses that to create the initial part of the hover text. SCIP indexers should not repeat the text in signature_documentation inside the documentation field.