chore: Formatting notebooks

[scart97]

This pull request has the objective to clean small formatting problems on the tutorial notebooks so they render better on the docs page, and also remove some references to things from fastai v1 that don't make sense anymore, like using the full 10 speakers dataset when we only need one audio file.

The only code change was updating the link of 10 speakers sample. As both it and ESC-50 downloaded a file called master.zip, there could be problems where both files got mixed in the fastai cache and you end up with the wrong dataset.

For the notebook changes:

ESC-50:
* Removed empty cells
* Save output of show_batch and the training so users know what to expect while reading the tutorial
* Fix headings so Table of Contents is rendered correctly

Training tutorial:
* Removed, it's exactly the same file as ESC50

Introduction to fastaudio:
* Change data to 10 speakers sample from full 10 speakers

Introduction to audio:
* Removed Table of contents, as the docs page creates one
* Fix headings
* Remove reference to notebooks that don't exist
* Change data to 10 speakers sample from full 10 speakers

[review-notebook-app]

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[scart97]

The rules for the toc rendering correctly are the follow:
The first cell should be a h1 heading (# Title)
All of the following cells that are headings should be h2 or more (## Section heading)

[scart97]

Example of the automatic TOC rendering correctly:

[codecov]

Codecov Report

Merging #73 (7534d6e) into master (d6d61aa) will decrease coverage by 0.30%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #73      +/-   ##
==========================================
- Coverage   90.88%   90.57%   -0.31%     
==========================================
  Files          12       12              
  Lines         647      647              
  Branches       70       70              
==========================================
- Hits          588      586       -2     
- Misses         32       34       +2     
  Partials       27       27              

Impacted Files	Coverage Δ
src/fastaudio/core/signal.py	81.25% <ø> (-2.09%)	⬇️

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[github-actions]

🎉 This PR is included in version 0.1.1 🎉

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