chore: replace some pngs with mermaid (#272)

Just replaces some PNGs with mermaid.

This is nice because they can change color for the website, and they are
more rigorous and maintainable (we already use mermaid in the spec).

old:


![image](https://github.com/user-attachments/assets/de15b19b-aa3b-41b7-a264-8c64897de1e1)

new: 

```mermaid
flowchart LR
    A[Application] --> API(Evaluation API)
    API --> P[Provider]
    P --> FMS[(Flag Management System)]
```


![image](https://github.com/user-attachments/assets/f90853f9-7211-4c1a-a10a-e66aeb21b501)

```mermaid
flowchart LR
    B(('Before' stage)) ---> FE[Flag Evaluation]
    B -..->|Error| E
    FE ---> A(('After' stage))
    FE -..->|Error| E(('Error' stage))
    A ---> F(('Finally' stage))
    E -..-> F
```

Signed-off-by: Todd Baert <[email protected]>

Author: toddbaert