Create Solution Children “join”

This MR migrates the Open Source Join solution child page to a new template.

/solutions/open-source/join/

It' also the 3nd iteration of the solutions template and makes a few tweaks:

• Addition of a marketo form block
• solutions.css added to template

To QA, make sure to check existing solutions pages for regressions:

/solutions/open-source/
/solutions/public-sector/
/solutions/open-source/partners/

We are doing this iteration to improve the solutions template and make it more usable for partners to self serve updates using Netlify CMS.

Related https://gitlab.com/gitlab-com/marketing/inbound-marketing/growth/-/issues/1432