Fix links to Mattermost Boards documentation + some others

In the meantime, some links to docs on mattermost boards seem to have been abandoned.

So, replacing the links to somewhat working ones ...although not maintained anymore

Okay so far @hueser93 ?

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fixing more

• https://eosc-portal.eu/
• https://aarc-project.eu/policies/policy-development-kit
• https://aarc-project.eu/policies/snctfi/
• https://aarc-project.eu/policies/sirtfi/