Update URLs for DM and Dumpling

[dveeden]

These move to a different repo on GitHub.

Should these be GitHub URLs? Or maybe the docs are more useful?

In case we want the docs instead:

• https://docs.pingcap.com/tidb/stable/dumpling-overview
• https://docs.pingcap.com/tidb/stable/dm-overview

[dveeden]

Might be useful to check this for more repos with something like this:

#!/bin/bash
for repo in $(grep -E '^- .*github.com/' README.md | sed 's#^.*https://github.com/##' | cut -d\) -f1 | sort)
do
    a=$(curl -s https://api.github.com/repos/${repo} | jq '.archived')
    echo -e "$repo: archived=$a"
done

Note that without authentication the API limit is quite low.