Update simple commands in MD and minor syntax fix sites/handbook/source/handbook/markdown-guide/index.html.md

Why is this change being made?

adding clarification on underlined text and usage of bold and italic text together at the same time as it took me a while researching this outside of the handbook to figure out. Also removing a wrong syntax that resulted in minor strange text artifacting on the handbook. for reference kindly check this article for italic and bold together and this post for underlined text. both methods are tried and tested. for syntax error issue, kindly check attached screenshots.

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
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