[docsy] Keep things DRY - avoid copy-pasted content from other repos

[chalin]

This is a transfer of:

• Keep pages DRY DarikshaAnsari/in-toto-docs#22

in particular this comment DarikshaAnsari/in-toto-docs#22 (comment):

We should not have the same content in two places. A solution will need to be found. We could pull in the sources from the other repo's at build time

At the moment, in the new upcoming version of the website, the following pages have some or all of their content copied from other repos:

• Ecosystem adopters and integrations, which comes from in-toto/friends. Related:
  • Reorganize the content of this repo so that it can be published directly on the website friends#39
• Overview, which comes from the spec. Related:
  • [docsy] Consider hosting a version of the spec directly on the website #72
• Getting started page is a copy of https://github.com/in-toto/in-toto/blob/develop/README.md