chore: consolidate Slack info in top-level policy doc

[erickzhao]

It's confusing that we have three separate Slack access docs, which leads to unnecessary fragmentation of information.

This PR moves all information into a single source of truth under the policy/slack.md doc.

It also updates some stale information accordingly.

[nikwen]

Thank you so much, Erick!

[nikwen]

Suggested change

	Before a Guest is added to a channel, a message must be posted to that channel stating the intent to
	#### Adding Guests to other channels than the default channels
	Before a Guest is added to a channel, a message must be posted to that channel stating the intent to

Suggestion

[nikwen]

Lint complains with broken links:

File Location: policy/slack.md
	Broken link on line 48: File does not exist at path: /home/runner/work/charter/README.md
	Broken link on line 82: File does not exist at path: /home/runner/work/charter/README.md
File Location: wg-infra/policy/access/README.md
	Broken link on line 9: File does not exist at path: /home/runner/work/governance/governance/wg-infra/policy/access/slack.md

[erickzhao]

In the last few commits, I separated it into two docs for consistency with the wg-infra/policy/ folder format.

• Anything involving etiquette belongs in the top-level doc.
• Anything involving access roles belongs in the infra policy.

Both docs now link to each other to avoid information being lost.

[erickzhao]

so powerful