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/

assigned to @jandt-desy

requested review from @hueser93

changed title from Fix links to Mattermost Boards documentation to Fix links to Mattermost Boards documentation + some others

changed the description

added 3 commits

• eb2bab1b - EOSC
• 8e18d5cb - AARC PDK
• cafef56a - AARC

Compare with previous version

marked the checklist item https://eosc-portal.eu/ as completed

marked the checklist item https://aarc-project.eu/policies/policy-development-kit as completed

marked the checklist item https://aarc-project.eu/policies/snctfi/ as completed

marked the checklist item https://aarc-project.eu/policies/sirtfi/ as completed

changed the description

merged

/cc @hueser93 @frust45 merging this to get rid of 404 links in our doc ;)

If you have better replacement links for the mattermost related stuff, of course, feel free to change again!

mentioned in commit a66db943