Third party PDF libraries must now be installed via composer and naturally
via composer autoloading mechanism. Because of that it is not necessary
to specify their path on disk. The usage is simplified and it allows us
to include them in our unit tests.
This also means that from now on PhpSpreadsheet must use composer autoloader
mechanism. The internal autoloading implementation was dropped.
We wrapped lines to make easier the source easier to read, and to have more manageable diffs.
This was done with something like:
```sh
pandoc --wrap=auto --atx-headers -f markdown -t markdown-fenced_code_attributes+pipe_tables+raw_html+intraword_underscores`
```
We aim for long pages instead of several small one, to make
`CTRL+F`ing easier and URL less prone to changes in the future.
There is still way to improve it though...
