docs: migration guide for the grouped render-parameter API

[MAfarrag]

Context

Part of the grouped render-parameter refactor (branch `refactor/grouped-render-params`, PR #274). Because the
loose-keyword shims are removed rather than kept, downstream users need a clear migration path.

Problem / Current Behaviour

docs/migration.md covered only the earlier subpackage restructure. There was no guidance for the grouped
render-parameter changes (removed keywords, new required object types, renames).

Affected locations

File	Symbol	Notes
docs/migration.md	"Grouped render-parameter objects" section (new)	before/after + mapping tables
mkdocs.yml	nav	add the Migration guide page

Proposed Solution

Add a "Grouped render-parameter objects" section with mapping tables for PointOverlay, FrameLabel,
cell_value_text_colors, facet(labels=...) / figure_size, num_domain_cells, and the colour / scale / cell
groups; restructure the page into two labelled migrations and wire it into the mkdocs nav.

Out of Scope

The code changes themselves (tracked in their own issues).

Effort Estimate

Size: S
Rationale: one docs page section plus a nav entry.

Definition of Done

• migration guide covers every removed/renamed API with before/after
• page is in the mkdocs nav and builds
• all existing tests continue to pass