docs(apify-actor-development): require explicit .inputSchema reference in actor.json (auto-discovery deprecated)

[DaveHanns]

Rationale

The current ## Input schema section in skills/apify-actor-development/SKILL.md links to references/input-schema.md but does not surface a common pitfall in SKILL.md itself: when an input schema file is created, .actor/actor.json must reference it explicitly via the inputSchema field. Relying on the historical auto-discovery of .actor/INPUT_SCHEMA.json or root INPUT_SCHEMA.json is deprecated per the input-schema specification and may be removed in a future platform release. Some tutorials and templates still show file creation without the linkage — that omission is the gap, not the recommended pattern.

This adds a short paragraph so agents following SKILL.md wire the schema up correctly the first time, instead of producing Actors that appear to configure input but fall through to a deprecated fallback.

Added content (verbatim)

When you create an input schema file, reference it explicitly from .actor/actor.json — either inline ("inputSchema": { "type": "object", ... }) or by path ("inputSchema": "./input_schema.json", relative to .actor/). Do not rely on auto-discovery of .actor/INPUT_SCHEMA.json or root INPUT_SCHEMA.json; that fallback is deprecated per the input-schema specification and may be removed in a future platform release. The deprecated .input field is still recognized for backwards compatibility but should not be used in new Actors.

Surfaced during an evaluation of Apify surfaces for agent-driven Actor development.