docs: Add Transaction Service guides

[falvaradorodriguez]

Closes safe-global/safe-transaction-service#1784

[github-actions]

Overall readability score: 68.26 (🔴 -0.01)

File	Readability
api.md	67.82 (-)

View detailed metrics

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

File	Readability	FRE	GF	ARI	CLI	DCRS
api.md	67.82	53.78	8.72	8.9	11.34	7.43
	-	-	-	-	-	-

Averages:

	Readability	FRE	GF	ARI	CLI	DCRS
Average	68.26	58.65	9.22	9.94	10.82	7.03
	🔴 -0.01	🔴 -0.12	🟢 +0.01	🟢 +0.03	🔴 -0.01	🔴 -0.01

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

[Uxio0]

Looks really good, but I left some questions

[Uxio0]

Really good job

[Uxio0]

Please let the team know in #general after this is deployed