docs: document legacy→beta token migration recipe for /next components

[pomfrida]

No doc describes how to migrate a /next component from legacy --eds-color-* tokens to the new beta tokens (the --_tone-N pattern used in the Button/Chip migrations #5222/#5225).

• Document the recipe in documentation/agent-instructions/BUILDING_EDS_2_COMPONENTS.md (or a new how-to), including the --_tone-N convention and how to look up the new variable names.

Blocked until #5222/#5225 land the pattern on main — don't write it before the convention is stable.

[claude]

These issues look related and may be useful context for writing the migration recipe:

• [Button] Migrate to new beta colour tokens #5208 — [Button] Migrate to new beta colour tokens (open): the tracking issue behind PR refactor: migrate Button (next) colours to beta tokens #5222; it is the designated test case for the migration pattern and its Definition of Done includes "learnings captured to guide the remaining component migrations" — i.e. the raw material this doc should distil.
• [Chip] Migrate to new beta colour tokens #5223 — [Chip] Migrate to new beta colour tokens (open): the tracking issue behind PR refactor: migrate Chip (next) colours to beta tokens #5225; explicitly describes the --_tone-N indirection pattern established in Button plus which semantic groups (selected, border-focus, canvas) were used, so it documents the convention this issue wants written up.
• 🧩 Component migration (colour, then spacing & typography) #5119 — 🧩 Component migration (colour, then spacing & typography) (open): the parent epic for migrating all EDS 2.0 components onto the redefined tokens; a documented recipe here would serve every remaining component in that epic, and phase 2 (spacing & typography) suggests the doc may need extending later.

This comment was generated by an LLM and may be inaccurate. Please verify that the linked issues are actually relevant.