DOC-1080: Update Mattermost credentials to medium tier template

[freakwriter]

Remove prerequisites section, convert bullet point list to more detailed numbered instructions, move the various steps for enabling personal access tokens site-wide/for individual users to separate section for readability and more detail.

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[github-actions]

Overall readability score: 43.22 (🟢 +0.02)

File	Readability
mattermost.md	60.26 (🟢 +22.7)

View detailed metrics

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

File	Readability	FRE	GF	ARI	CLI	DCRS
mattermost.md	60.26	35.13	7.75	11.6	13.78	7.05
	🟢 +22.7	🟢 +17.12	🟢 +2.36	🟢 +3.2	🟢 +3.25	🟢 +2.05

Averages:

	Readability	FRE	GF	ARI	CLI	DCRS
Average	43.22	34.92	11.66	14.86	14.34	8.53
	🟢 +0.02	🟢 +0.01	🟢 +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