Improve bug severity definitions and consolidate guidance for clarity

What does this MR do?

Improve the definitions of Severity and how to use them.

• We have 2 tables which can be confusing, removed the 2nd table.
• Emphasize the impact guidance, merged them into a single table.
• Remove the examples from the table into a list.
• Make the examples clearer.
• Removed the confusing excerpt on priority "If an issue seems to fall between two priority labels, assign it to the higher-priority label."

@clefelhocz1 @marin @at.ramya @tpazitny on to you for review first.