ci: only check new links on pull requests to avoid rate limiting

[MikeGoldsmith]

Description

The check-links CI job checks all links in changed markdown files. For files like CHANGELOG.md that contain hundreds of URLs, this triggers HTTP 502 rate limits from GitHub, causing spurious CI failures across many PRs.

Changes

• Pull requests: only URLs from added lines in the diff are extracted and checked. This avoids re-checking hundreds of existing links that haven't changed.
• Push to main: unchanged — all links in changed files are still checked for full validation.
• Added fetch-depth: 0 to the checkout step so git diff has access to the base branch.

How it works

git diff origin/$base...HEAD -- <changed files>
  | grep '^+'           # only added lines
  | grep -v '^+++'      # skip diff headers
  | grep -oP 'https?:…' # extract URLs
  | sort -u             # deduplicate

The extracted URLs are written to a temporary markdown file and passed to markdown-link-check.

Mirrors open-telemetry/opentelemetry-python#5162 which reduced the check-links job from 17 minutes to 20 seconds.