Add guides to documentation

[seanmor5]

Here is an (incomplete) list of guides that should be added to the documentation:

• Basic Axon models
• Complex Models (combinators, weight sharing, etc.)
• Working with Named Tensors (requires Integrate named tensors #57)
• Training and Evaluating Axon models
• Creating Custom Layers
• Custom Training Loops
• Creating Custom Event Handlers and Metrics
• Mixed Precision Training
• Axon Graph Manipulation (requires Introduce public graph manipulation API #117)
• Pruning trained models (requires Add model pruning #120)
• Model Serialization and Saving (requires Add model import/export API #26)
• Hooks (requires Add training hooks #77)
• Axon Optimization Deep Dive
• Transfer Learning and Fine Tuning

Note these differ from examples of specific applications of Axon as they are intended to be example based deep dives of certain aspects of the API.

[josevalim]

Just a note: Livebook supports links in more recent versions, so you can have a root guide that points to all other guides. :)