Add API reference and Change Log

[jwhhh]

We need an API Reference page for the EspressoProblem class, EspressoError exceptions and other utility functions.

About the change log, each example can have its own and we check which contributions are changed and which are newly added

(in CHANGELOG.md)

# Change Log

## VERSION NUMBER (DATE)

### New examples

- ...
- ...

### Changed examples

- ...

### Core utilities

- ...