Add section on how to test UI

[kdheepak]

[joshka]

Some source material:

• https://discord.com/channels/1070692720437383208/1072907135664529508/1194233004403474512
• https://discord.com/channels/1070692720437383208/1072907135664529508/1192608138818506752
• https://discord.com/channels/1070692720437383208/1072907135664529508/1192608924520689715
• https://discord.com/channels/1070692720437383208/1072907135664529508/1192768852937949234
• https://discord.com/channels/1070692720437383208/1072879985762762812/1191548383605837944
• https://discord.com/channels/1070692720437383208/1070692720437383211/1180191813382963220

[kdheepak]

As @EdJoPaTo pointed out:

Should probably wait until ratatui-org/ratatui#1007 is settled.