docs: sync API v2 reference with fields param and alias removal

[Program2113]

Summary

Updates the API v2 reference on top of #303 to match the current OpenAPI document in plane-ee (apps/api/plane/api_v2/core/schema/openapi/).

What changed

1. ?fields= sparse fieldsets (new)

   • New concept guide: /api-reference/v2/fields
   • Omit semantics, all token, list deferral, detail-only fields
   • Documented on list/get/create/update pages and wired into the sidebar

2. Alias removal

   • Path segments address by stable keys only (UUID + workspace slug, bare project identifier, work-item PROJ-123)
   • No scheme-prefixed aliases (name:…, key:…, external:…)
   • Attribute lookup via list identity filters (?name=, ?external_id=) + optional ?fields=id
   • Documented on filtering guide, migration guide, and states/labels/cycles/modules list pages

3. ?expand= accuracy

   • Work items now: state, type, parent, assignees, labels, cycle, modules
   • Also documented for comments (actor), cycles (owned_by), modules (lead, members)
   • Removed stale note that expand was absent from OpenAPI

4. Work item shape fixes

   • custom_fields is detail-only (absent on list rows, not null)
   • Read shape includes project_id, cycle_id, module_ids
   • Parent project_id path segments accept UUID or bare project identifier

Source of truth

Verified against apps/api/plane/api_v2/core/schema/openapi/ and the fields-support design in plane-ee.

Relation to #303

This PR targets the docs/api-reference-v2 branch so it can be merged into #303 (or cherry-picked). Diff is the single follow-up commit on top of #303.

Test plan

• Preview sidebar shows Sparse Fieldsets under v2 Concepts
• /api-reference/v2/fields renders with standard doc layout (not wide API layout)
• Migrating from v1 no longer claims "There is no ?fields= in v2"
• Work item list examples omit custom_fields key
• Expand tables list cycle / modules for work items

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