NPCs documentation cleanup

[cake-pie]

Summary

None

Purpose of change

Clean up organization and formatting of NPCs.md, part 1/?.

Describe the solution

"type": "npc", documentation:

• fix missing paragraphing
• code formatting
• remove reference to obsoleted NPCATT_DEFEND (see NPC vs NPC combat #17781)
• merge info about age and height fields, which do not deserve their own top-level heading

Special Custom Entries

• fix missing table formatting

Talk topics, Responses

• add links to other sections for "see ... for details" text

Responses

• group condition, switch and default together under "Response Availability" as these control which responses are displayed to the player as possible responses
• move section about text field so that the common basic case is discussed first, before truefalsetext is introduced as a possible substitute
• move misplaced sample trials
• reorganize "success and failure" so that content about each field is consolidated together rather than interleaved among one another
• merge semi-redundant "opinion changes" into a subsection of "success and failure"
• Undo text changes regarding mission_opinion from doc/npcs.md: fix some bugs in the formatting #28507, after confirming that relevant code from npctalk: add a bunch of new options to JSON #27734 / cf02075 is still intact: 1 2 3