docs: survey vars multi-select, targets, and executor image

[cursor]

Summary

Bumps the semaphore-docs submodule with user-guide updates for recently shipped template features.

Docs added/updated

Doc	Changes
survey-vars.md	Text and select types, default values, pass-variable-as (env vs extra-var), JSON encoding for multi-select with shell/PS/Python examples
task-templates/README.md	Per-template executor image override for Docker/K8s runners

Companion PR in semaphore-docs.

Codepaths covered

• db/Template.go — SurveyVarSelect, SurveyVarTarget, SurveyVarDefaultValue, ValidateSurveyVar
• services/tasks/local_executor.go — formatVarValue, getSurveyEnvVars, env-target exclusion from extra-vars
• db/Template.ExecutorImage — per-template container image override (migration v2.19.12)

Key knowledge gaps addressed

• Multi-select survey variables delivered JSON-encoded arrays; previously undocumented
• Survey variable target: env vs default CLI/extra-vars delivery was missing from user docs
• Template-level executor_image override had no user-facing documentation despite UI and API support