META: Fix outdated screenshots and steps in Fleet & Agent docs (reported by QA)

[kilfoyle]

Here's the list of issues:

• [Fleet]: Screenshot not updated for standalone agent installation. #1231 -> Fix screenshot: 1231 #1581
• [Fleet]: Screenshot not updated for Agent policies tab. #1232 -> Fix screenshot: 1232 #1582
• [Fleet]: Screenshot not updated for integration Tab. #1233 -> Fix screenshot: 1233 #1583
• [Fleet]: Screenshot not updated for Index Management. #1234 -> Fix screenshot: 1234 #1585
• [Fleet]: Screenshot not updated for creating Agent policies. #1235 -> Already fixed by Update docs for reusable integrations policies #1264
• [Fleet]: Screenshot not updated for Agent logs tab as Reset to policy link is not available. #1236 -> Already fixed.
• [Fleet]: Screenshot is outdated for Add agent flyout as unprivileged agent details are missing. #1237 -> Fix screenshot: 1237 #1586
• [Fleet]: Steps not updated for Download Elastic Agent diagnostics. #1238 -> Fix steps and screencap for agent diagnostics: 1238 #1587
• [Fleet]: Screenshot not updated for edit fleet server integration. #1239 -> Fix screenshot: 1239 #1588
• [Fleet]: Screenshot not updated for Fleet Server Policy setting page #1240 -> Fix screenshot: 1240 #1589