[WiP] Docs restructuring, DisCoCirc material

[dimkart]

With the opportunity of the upcoming lambeq release 0.5 and the addition of the new material, this PR also attempts a major restructuring of lambeq docs. (@neiljdo also contributes to this work.)

• Create new basic structure (see separate comment below)
• Organise material in folders
• Check lambeq's source code for broken references to images due to restructuring (Documentation-related changes for new release (docstrings) lambeq#199)
• Re-organise "Getting Started" Section
• Add a quick-start tutorial
• Add dedicated Support section in menu
• Move necessary material to User Guide
• Add a "lambeq and compositionality" page
• Add some introductory text to new TOC pages that have been created for the restructuring.
• Re-organise training tutorials
• Remove UML and redundant API information
• Re-organise auto-doc features for API
• Create new datasets for DisCoCirc
• Delete root-API file
• Add DisCoCirc basics tutorial
• Add DisCoCirc training tutorial
• Update glossary
• Update bibliography
• Adjust old material with references to new glossary terms, new parers in bibliography etc.
• PregroupTreeNode example in docstring does not work, fix in lambeq (Documentation-related changes for new release (docstrings) lambeq#199)
• Add pregroup tree API information
• Tests currently failing because lambeq is installed from pip. Need to check after release.
• Navbar links should be updated in quantinuum-sphinx repo

[dimkart]

New (tentative) structure here:

GETTING STARTED

• What is lambeq?
• Installation
• Quickstart

USER GUIDE

• lambeq and compositionality
• Pipeline
• String diagrams
• Syntactic parsing
• Machine learning with lambeq
• Command-line interface

TRAINING

• From sentence to circuit
  • Step 1. Sentence input
  • Step 2. Diagram rewriting
  • Step 3. Parameterisation
• From text to circuit (DisCoCirc)
• ML training tutorials
  • Training: Classical case
  • Training: Quantum case
  • Training: Hybrid case
  • Training: DisCoCirc
  • Manual training
• Advanced: low-level lambeq
  - Monoidal categories in lambeq
  - DisCoCat in lambeq
• Code examples
• NLP-101

API

• lambeq.ansatz
• lambeq.backend
• lambeq.bobcat
• lambeq.experimental
• lambeq.rewrite
• lambeq.text2diagram
• lambeq.tokeniser
• lambeq.training

SUPPORT

• User support
• Troubleshooting
• Contributing to lambeq
• How to cite
• Licence
• Release notes

REFERENCE

• Glossary
• Bibliography
• Index

EXTERNAL LINKS

• Resources
• Web demo
• DisCoPy