docs(links): update README to reflect current link checkers

[MeGaurav4]

Closes #45508

What?

Updated src/links/README.md to match the current link-checking system:

• Replaced the removed rendered-content-link-checker.ts script and the
  unavailable rendered-content-link-checker-cli npm command with the current
  check-links-pr, check-links-internal, and check-links-external scripts.
• Removed the obsolete --level and --check-external-links options and
  documented the real options (--files, --version/--language, --max).
• Added the components/ path to the LinkPreviewPopover.tsx reference.
• Updated the stale performance note that still named the removed checker.

Why?

The issue reporter noticed the README still describes the previous
rendered-content link checker while working on #45507. The deleted script and
command no longer exist, so the docs were misleading.

Checks

• No rendered-content-link-checker references remain (grep verified)
• All documented scripts/commands verified against src/links/scripts and package.json

[github-actions]

👋 Hey there spelunker. It looks like you've modified some files that we can't accept as contributions:

• src/links/README.md

You'll need to revert all of the files you changed that match that list using GitHub Desktop or git checkout origin/main <file name>. Once you get those files reverted, we can continue with the review process.

The complete list of files we can't accept are:

• .devcontainer/**
• .github/**
• data/reusables/rai/**
• Dockerfile*
• src/**
• package*.json
• content/actions/how-tos/security-for-github-actions/security-hardening-your-deployments/**

We also can't accept contributions to files in the content directory with frontmatter contentType: rai.

[github-actions]

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[MeGaurav4]

Closing: the triage check (check-unallowed-file-changes) blocks all external contributions touching src/** (see src/workflows/unallowed-contribution-filters.yml). This change can only land from the internal repository. The issue reporter noted this while working internally on #45507, so leaving the fix to the docs team. Thanks for flagging #45508.