docs: Update mission, vision, and roadmap

[vdemeester]

Changes

This PR modernizes the community roadmap.md:

Roadmap → Project Board

• Replace inline 2021/2023 goals with a pointer to the Tekton Community Roadmap v2 project board
• Add guidance on how items get added (via area/roadmap label or manually)
• Follows the same pattern as tektoncd/pipeline's roadmap.md

Updated Mission & Vision

• Refresh the mission statement to reflect current focus: CI/CD, supply chain security, and workflow automation
• Update the vision to include:
  • Pipelines as Code and event-driven workflows
  • Supply chain security (Chains, SLSA, signed provenance)
  • AI/ML pipelines and general-purpose workflows
  • Modern tooling ecosystem (dashboards, policy systems)

Archived Past Goals

• Move 2021 and 2023 goals into collapsible <details> sections for historical reference
• Promote per-project roadmap links into their own section

[tekton-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by:
To complete the pull request process, please ask for approval from vdemeester after the PR has been reviewed.

The full list of commands accepted by this bot can be found here.

Details

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment