…but is now easier to change by changing there and {{ site.data.variables.contact_url }} which is probably easier for most than running a script with a sed command that is hard to figure due to character escapes:
find . -type f -name "*.md" -print0 | xargs -0 sed -i '' -e 's,https://bit.ly/arc-help,{{ site.data.variables.contact_url }},g'
…but is now easier to change by changing there and {{ site.data.variables.contact_url }} which is probably easier for most than running a script with a sed command that is hard to figure due to character escapes:
find . -type f -name "*.md" -print0 | xargs -0 sed -i '' -e 's,https://bit.ly/arc-help,{{ site.data.variables.contact_url }},g'