provider-docs: add action documentation guidance

[AdamTylerLynch]

The provider-docs skill covered resources, data sources, ephemeral resources, list
resources, functions, and guides — but not actions, so an agent invoked to
document an action got no guidance, even though the sibling provider-actions skill
points here for the docs workflow.

Verified before writing (sources checked, not assumed):

• The Terraform Registry docs format supports docs/actions/<action>.md pages
  (developer.hashicorp.com/terraform/registry/providers/docs)
• tfplugindocs generates action documentation — including scaffolding missing
  action templates — with Terraform v1.14.0+, using
  templates/actions/<action type name>.md[.tmpl] and example files at
  examples/actions/<action_type>/action*.tf (hashicorp/terraform-plugin-docs)

Changes

• SKILL.md: actions added to the trigger description, the doc-target list, and the
  template-path list (with the v1.14.0+ note), plus a pointer to the new reference
  section for action-page structure
• references/hashicorp-provider-docs.md: new Action Pages section anchored on
  how terraform-provider-aws documents its actions (website/docs/actions/, 11
  hand-written pages analyzed):
  • # Action: <name> H1 prefix, parallel to # Resource: / # Data Source:
  • Examples must show both the action block and the resource-side
    lifecycle { action_trigger { ... } } wiring — an action example without a
    trigger is not runnable
  • ## Argument Reference for config arguments; no attribute/output section
    (actions produce no state — every AWS action page omits it)
  • Callout conventions while actions are experimental: ~> **Note:** alpha/preview
    disclaimer and !> **Warning:** when the action mutates state Terraform does
    not reconcile (the aws_ec2_stop_instance precedent)
  • release-note:new-action changelog type for repos using go-changelog
  • Notes AWS hand-writes these pages (it predates tfplugindocs action support);
    new providers should generate instead

Verification: scripts/validate-structure.sh passes; two-file diff confined to
the provider-docs skill; no conflicts with the other six branches (none touch
provider-docs).

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The follow-up commit also captures the documentation-example conventions requested in #14: examples live in examples/ (one per file, imported via tffile) and must not contain terraform, provider, or output blocks.