docs(web-server-schema): call out OpenAPI 3.0.x requirement and add 3.1 to 3.0 migration page

[DaveHanns]

Summary

• Update the web server schema landing page to state OpenAPI 3.0.x explicitly (the build validator rejects 3.1) and link to a new compatibility page.
• Add a new page under the web server schema section that lists common OpenAPI 3.1 constructs that break the build alongside the 3.0.x form to use instead, with a full nullable example and two local validation commands.

Motivation

webServerSchema in .actor/actor.json is validated against OpenAPI 3.0.x at build time. The current docs describe the field as accepting "OpenAPI 3.x" and link out to the 3.0.3 spec, but never state that 3.1 is not accepted. Users who copy an existing spec generated by a tool that defaults to 3.1 - or hand-write one using the 3.1 nullable form "type": ["string", "null"] - see the build fail with a validation error that gives no hint about the version constraint or how to fix it.

Evidence

Minimal reproducer - an .actor/openapi.json that is valid OpenAPI 3.1 but uses only 3.1-specific syntax:

{
    "openapi": "3.1.0",
    "info": { "title": "My API Actor", "version": "1.0.0" },
    "paths": {
        "/items": {
            "get": {
                "responses": {
                    "200": {
                        "description": "OK",
                        "content": {
                            "application/json": {
                                "schema": {
                                    "type": "object",
                                    "properties": {
                                        "name": { "type": ["string", "null"] }
                                    }
                                }
                            }
                        }
                    }
                }
            }
        }
    }
}

apify push triggers a build that fails validation of webServerSchema with no indication that the root cause is the OpenAPI version. The existing docs at /platform/actors/development/actor-definition/web-server-schema do not mention the constraint, so there is no obvious next step for the user other than trial-and-error.

Changes

• sources/platform/actors/development/actor_definition/web_server_schema/index.md: replace two "OpenAPI 3.x" mentions with "OpenAPI 3.0.x", and link to the new compatibility page from both the intro and the "Define the web server schema" section.
• sources/platform/actors/development/actor_definition/web_server_schema/openapi_version.md (new): explains the version constraint, shows the correct nullable: true form, includes a table of common 3.1 to 3.0 translations (nullable, exclusiveMinimum/exclusiveMaximum, const, examples, $schema, webhooks, contentMediaType), a full nullable example, and two local validation commands (@redocly/cli and @apidevtools/swagger-cli).

Test plan

• pnpm start renders the new page under Web server schema in the sidebar.
• Both links added to the landing page resolve to the new page.
• The nullable JSON example on the new page validates against OpenAPI 3.0.3 with npx @redocly/cli lint.

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

[apify-service-account]

✅ Preview for this PR (commit f937a04) is ready at https://pr-2734.preview.docs.apify.com (see action run).