Creates policy plan index

[clausmullie]

in order to enable archiving old one, and enable #1401

[clausmullie]

If actions comes up with the error

 - ./_site/organization/general-assemblies/minutes-28-06-2021.html
  *  internally linking to ../policy-plan.md, which does not exist (line 184)
     <a href="../policy-plan.md">our policy plan 2019-2022</a>

Does that mean the redirect from isn't working well?

[Ainali]

Does that mean the redirect from isn't working well?

Well, the redirect from organization/policy-plan works well (as well as any external link pointing to https://about.publiccode.net/organization/policy-plan.html ), but the link in the minutes points to the .md file. So when a page is moved, we should fix all old links within the site as they will not become a html link anymore.