mkt: add slack chat banner

[alerizzo]

👀 Live preview

Added it in 2 places:

• Getting Started / Quick Start: https://slack-chat-banner--docs-codacy.netlify.app/getting-started/codacy-quickstart/
• Change plan and billing: https://slack-chat-banner--docs-codacy.netlify.app/organizations/changing-your-plan-and-billing/

🚧 To do

• If relevant, include the Jira issue key at the end of the pull request title
• Perform a self-review of the changes
• Fix any issues reported by the CI/CD

[github-actions]

Overall readability score: 53.69 (🟢 +0)

File	Readability
codacy-quickstart.md	50.64 (🟢 +0)
changing-your-plan-and-billing.md	44.04 (🟢 +0)

View detailed metrics

🟢 - Shows an increase in readability
🔴 - Shows a decrease in readability

File	Readability	FRE	GF	ARI	CLI	DCRS
codacy-quickstart.md	50.64	38.21	11.69	12.5	13.22	7.94
	🟢 +0	🟢 +0	🟢 +0	🟢 +0	🟢 +0	🟢 +0
changing-your-plan-and-billing.md	44.04	29.38	11.97	16.6	13.64	7.35
	🟢 +0	🟢 +0	🟢 +0	🟢 +0	🟢 +0	🟢 +0

Averages:

	Readability	FRE	GF	ARI	CLI	DCRS
Average	53.69	42.87	10.84	12.54	12.6	7.94
	🟢 +0	🟢 +0	🟢 +0	🟢 +0	🟢 +0	🟢 +0

View metric targets

Metric	Range	Ideal score
Flesch Reading Ease	100 (very easy read) to 0 (extremely difficult read)	60
Gunning Fog	6 (very easy read) to 17 (extremely difficult read)	8 or less
Auto. Read. Index	6 (very easy read) to 14 (extremely difficult read)	8 or less
Coleman Liau Index	6 (very easy read) to 17 (extremely difficult read)	8 or less
Dale-Chall Readability	4.9 (very easy read) to 9.9 (extremely difficult read)	6.9 or less