Correct Slack URL references

[planetf1]

In egeria-docs we have some issues with our slack URLs

• In some cases we link directly to a channel or archive - for example https://lfaifoundation.slack.com/archives/C01F40J2XA8 - but this only works if the user has already signed up to slack.
• We still have references to slack.odpi.org
• For joining we use https://slack.lfai.foundation, but this is returning an error. On checking with the lfaidata team, it seems the correct URL has been changed to https://slack.lfaidata.foundation

Links (search for 'slack') need to be checked, contexualized & updated.

NOTE: There are also a few references in other repositories such as base egeria & potentially on docker/quay.io which also need updating

• Fix slack references in egeria-docs
• Fix slack references for container images on quay.io & docker.io
• Fix slack references in core egeria
• Check other repos/places for references and fix