Enhancements: Back to ToC links + other minor adjustments

[dami-i]

• Adds "Back to Table of Contents" links on every section for navigation improvement;
• Moves a duplicate "step 4" on Visa payments to a side note;
• Standardizes the presentation for most of the steps sequences to bold text with colon as separator (e.g. "Step 1:");
• Formats Linux commands as monospace;
• Promotes a few texts to h4 headers;
• Other minor adjustments.

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