PlantUML diagrams: use *.puml extension

[php-coder]

Follow-up to #1363

Let's use *.puml file extension, so editor plugins (like the one for VSCode) will properly recognize these files.

Another small improvement is to change a color of the lifelines blocks (see https://plantuml.com/en/sequence-diagram) to stand them out.

[php-coder]

Another improvement: use hide footbox (see https://plantuml.com/sequence-diagram)