docs: uncomment hidden upgrade documentation

[jhaayushkumar]

Description

Uncommented 80+ lines of critical upgrade documentation that was hidden behind a TODO comment. This makes CLI upgrade steps, version compatibility warnings, and upgrade considerations visible to users.

Change Type

/kind documentation

Changelog

NONE

Additional Notes

Fixes :- #745

[jhaayushkumar]

@artberger @Nadine2016 if possible have a look at this PR whether this is ready to convert into purple or not

[kristin-kronstain-brown]

This doesn't seem to be as simple as just uncommenting. The commented out information seems to need some more attention.

Issue 1: "## Step 2: Upgrade the CLI" is wrong in two ways

1. docs/snippets/cli-name.md resolves to glooctl, which is Solo.io's proprietary CLI, not kgateway's. kgateway's CLI is kgwctl.

2. kgwctl has no upgrade subcommand. Its documented commands are: analyze, check, completion, describe, get, help. There is no mechanism to upgrade kgateway via the CLI. The upgrade is Helm-only. The entire "## Step 2: Upgrade the CLI" section should be removed, not published.

Issue 2: "## Step 3: Upgrade kgateway" is an empty heading

The comment boundary (-->) was placed immediately before the Step 3 heading with no body content under it. After the uncomment, Step 3 renders as a heading with nothing under it, followed immediately by ## Prepare to upgrade {#prepare}. This is a dead-end section.

Issue 3: Duplicate "prepare" sections

The newly revealed "## Step 1: Prepare to upgrade" (checking dependencies, setting $NEW_VERSION) and the pre-existing "## Prepare to upgrade {#prepare}" serve the same purpose and now both appear in the same page, back to back.

Issue 4: export NEW_VERSION=... is set twice

Step 1 (step 3) sets it, and "## Upgrade {#upgrade-steps}" step 1 sets it again. A user reading end-to-end hits the same variable assignment in two different sections.