Added plantuml diagrams support

[DanZ-ix]

Summary

Added plantuml plugin, added plugin method process_md_before_html_convert(md: str) -> str, added dcoumentation

Details

Plantuml plugin support

New plugin can be used with tags [[plantuml start]] CODE [[plantuml end]], for example:

[[plantuml start]]
@startuml
Alice -> Bob: Authentication Request
Bob --> Alice: Authentication Response

Alice -> Bob: Another authentication Request
Alice <-- Bob: Another authentication Response
@enduml
[[plantuml end]]

This block is rendered using plantuml server (default is plantuml.com, custom can be defined with config) as an png diagram.
Plantuml code can be edited in .md file and after saving the diagram would be changed.
Two or more plantuml blocks are supported

New optional plugin method

Added new optional plugin method process_md_before_html_convert(md: str) -> str, that is called before converting .md file to html, but after saving it to the filesystem. It allows to make changes in md file that are not saved, but rendered in html.

Checks

• In case of new feature, add short overview in docs/<corresponding file>
• Tested changes

[Linbreux]

Hi @DanZ-ix,

Thats again a nice implementation. Did not have the time to test it yet.

Would it be possible to make it like the mermaid plugin, inside a codeblock? (I know there is no documentation on this one yet...). I guess that makes more sense to me.

```mermaid
graph LR
A[Square Rect] -- Link text --> B((Circle))
A --> C(Round Rect)
B --> D{Rhombus}
C --> D
```

So it would be

```plantuml
@startuml
Alice -> Bob: Authentication Request
Bob --> Alice: Authentication Response

Alice -> Bob: Another authentication Request
Alice <-- Bob: Another authentication Response
@enduml
```

[Linbreux]

LGTM, a few small changes need to be made and then its ready for merging. It worked great!