[HWORKS-3336] Add a Releases section for release notes, breaking changes and migrations

[robzor92]

Adds a top-level Releases section to the docs nav, grouped by major version, with one page per release (HWORKS-3336).

• docs/releases/index.md and docs/releases/5/index.md: section hubs.
• docs/releases/5/{5.1.0,5.0.1,5.0.0}.md: each page has Release notes, Breaking changes and Migrations, in that order.
• .claude/docs/releases.md: the writing convention, referenced from .claude/CLAUDE.md (AGENTS.md is a symlink to it).

The content of the three release pages is placeholder. It was picked from the helm and api GitHub release lists to show the layout, and it has not been verified. It must be replaced or checked before this merges.

Not covered yet (open questions on the ticket):

• The section lives on main only, so it shows in the dev docs until the next branch-X.Y is cut. Patch notes for older lines need a decision on where they're written.
• No PR template field or CI check enforces the convention.

hopsworks-docs markdownlint and hopsworks-docs check (strict) pass locally.

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