docs: propose ADR for generated SDK lifecycle

[krowvin]

Summary

This proposes ADR 0017 for the lifecycle of CDA's generated SDKs and adds it to the Design Decisions index. It uses the existing RST ADR structure with Summary, Proposal, Opinions, Decision Status, and References, and is marked proposed.

The proposal defines maintained generator source under clients/<language>, generation from the corresponding CDA OpenAPI export, cda-* names for new distributions, CDA-aligned release versions, development artifacts for untagged builds, package verification, executable examples, and shared GitHub Pages reference documentation linked from Read the Docs. It keeps higher-level clients separate and preserves the existing cwmsjs name/version scheme pending a separate migration decision.

Implementation is being reviewed separately in Python SDK #1925 and SDK Pages #1924. This PR changes documentation only and does not enable publishing or select a PyPI organization.

Validation

• Built the documentation with Sphinx and verified the new ADR HTML and its Design Decisions index link.
• The full documentation build reports 34 diagnostics in existing content, with none in the new ADR. These include existing malformed RST and missing references.
• git diff --check passed.