docs(versioning): fix stable-after-preview guidance for @previewVersion ordering

[Copilot]

The stable-after-preview how-to instructed authors to add a new stable version and remove @previewVersion later as a separate step. This produces a diagnostic because @previewVersion must only decorate the last version in the enum—adding a stable version after it violates that constraint.

Changes

• First step: Instruct authors to remove @previewVersion from the preview version and add the new stable version as the last enum entry (keeping the preview version so the spec still compiles during editing)
• Later step: Add explicit instruction to remove the old preview version from the enum after all decorator updates are complete

Corrected flow

// Step 1: Remove @previewVersion, add stable as last entry
enum Versions {
- @Azure.Core.previewVersion
  `2025-10-01-preview`,
+ `2026-01-01`,
}

// Step 2 (after decorator updates): Remove old preview
enum Versions {
- `2025-10-01-preview`,
  `2026-01-01`,
}

[markcowl]

Pull the latest from main, make these changes, ensure that all changed files are correctly formatted, this branch is up to date with the main branch and only the changes associated with this pr are in the pr diff.

[markcowl]

Here you should instruct the user to remove the @previewVersion decorator from the preview version and add the new stable version as the last entry in the enum.

If you remove the preview version, the spec will fail to compile, which will make editing difficult.

[Copilot]

Updated in the latest commit. The first step now instructs users to remove @previewVersion and add the new stable version as the last enum entry, keeping the preview version in place so compilation still works.

[markcowl]

Here, we should only instruct the user to remove the replaced preview version (since we will have removed the @previewVersion decorator above)

[Copilot]

Added a step here that instructs the user to remove only the replaced preview version from the enum (since @previewVersion was already removed in the first step).

[Copilot]

Pull the latest from main, make these changes, ensure that all changed files are correctly formatted, this branch is up to date with the main branch and only the changes associated with this pr are in...

Done. Pulled latest from main, made the requested changes, verified formatting is correct, and confirmed only the relevant doc file is in the diff.