Add Social Networks logos and link

We should add the links to our current social networks accounts on the footer:

• Mastodon: @OSPOAlliance@mastodon.opencloud.lu
• LinkedIn: https://www.linkedin.com/company/ospo-alliance/
• Twitter: @OSPOAlliance

Reading the doc and looking at other sites, I tried to add the following lines to the Params section of the config.toml files with no luck:

linkedin_url = "https://www.linkedin.com/company/ospo-alliance/"
twitter_url = "https://twitter.com/OSPOAlliance"

There is certainly more to do, but I can't figure out what. Furthermore, how can we add Mastodon?

Help is appreciated. Thank you.

/ping @cguindon