docs: Harmonise references to Cilium Slack

[qmonnet]

We link to Cilium Slack at multiple locations in Cilium's documentation. The way we do it is far from consistent:

• We usually provide a direct link to the Slack instance
• ... Which, over time, resulted in at least 13 occurrences of the same URL https://cilium.herokuapp.com
• Sometimes, we create an indirect link to a _slack target, pointing to the gettinghelp.rst page
• Some other times, we use other indirect links pointing to the glossary.
• At some locations, we also mention that users can get invites to the Slack instance. As far as I know, these invites are no longer necessary.

Let's try to clean this up.

• Create a target link in the RST epilogue in conf.py, so we can reference it from any location.
• Call this link everywhere we need, using `Cilium Slack`_ in a consistent fashion.
• Format most channel names as inline literals.
• Improve formatting (not the content) of some paragraphs that already get updates for Slack references: rewrap, remove trailing spaces, etc.

Marking for backports because it touches the doc framework (conf.py, check-build.sh).

No change to technical contents from the docs.

[qmonnet]

/test

[dylandreimerink]

LGTM

[lambdanis]

awesome