Update CD Rails design proposal with environment tiers

What does this MR do and why?

Update the GitLab CD Rails design proposal to document the environment-tier work shipped during Beta, as per the decisions made in https://gitlab.com/groups/gitlab-org/-/work_items/21247#note_3448765760.

The proposal previously described cd_environments with only name and description. This MR:

• Adds tier to the cd_environments schema in the ERD and the table description.
• Adds a new "Environment tiers" subsection explaining the design call: a fixed smallint enum on cd_environments.tier for Beta, replaced by a user-defined cd_environment_tiers table post-Beta, plus the migration plan.
• Links Environment in the entities list to the new subsection.

Details

The corresponding implementation has already landed:

• gitlab-org/gitlab!242621 (merged): add tier column and GraphQL surface.
• gitlab-org/gitlab!242835 (merged): add Organization.cdEnvironmentTiers listing query.
• gitlab-org/gitlab!242836 (merged): add tier filter on Organization.cdEnvironments.

The post-Beta follow-up (user-defined tiers, cd_environment_tiers table, management UI) is tracked in gitlab-org/gitlab#604327.

Author and Reviewer Checklist

Please verify the check list and ensure to tick them off before the MR is merged.

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added, and the content is SAFE
• Assign reviewers for this MR to the correct
  • The when to get approval handbook section explains when DRI approval is required
  • The who can approve handbook section explains how to identify the DRI
  • If the MR does not require DRI approval, consider asking someone on your team, such as your manager.
  • The approver may merge the MR. If they approve but don't merge, you can merge.
• For transparency, share this MR with the audience that will be impacted.
  • Team: For changes that affect your direct team, share in your group Slack channel
  • Department: If the update affects your department, share the MR in your department Slack channel
  • Division: If the update affects your division, share the MR in your division Slack channel
  • Company: If the update affects all (or the majority of) GitLab team members, post an update in #whats-happening-at-gitlab linking to this MR
    • For high-priority company-wide announcements work with the internal communications team to post the update in #company-fyi and align on a plan to circulate in additional channels like the "While You Were Iterating" Newsletter

Commits

• Update CD Rails design proposal with environment tiers

---