Add instructions to get page width and height

[vitormattos]

Based on #427 (comment)

Close #427

[k00ni]

@vitormattos Thank you for this pull request. It is a helpful addition to our docs.

If I understand your example code correct, it extracts height and width of each page not for the whole PDF. You referenced #427, which is about the width and height of the whole PDF. Based on the PDF specification there is no width/height of a PDF, but for each page.

Please ignore the failing CS job. I saw it failing in another PR. PHP-CS-Fixer changed its behavior lately, but we didn't make the change in master.

[vitormattos]

Yes @k00ni , is exactly this.
Is possible to exist a PDF file with pages with different sizes.
I used this package at LibreSign and put on my PR the reference of all changes that I did to use this package. My PR at LibreSign was a bit big because I removed the dependency of a binary in Go that was created only to get the dimensions of all pages.

[vitormattos]

rebased from master branch to make CI OK