Add "Introduction to Composer" translation

[Marqitos]

I add a page which did not yet have a Spanish translation.
Introduction to Composer

https://www.php.net/manual/es/install.composer.intro.php

[Marqitos]

Please Gina,

Can you explain how works the 2 lines:

<!-- EN-Revision: 3678a6db126048bf052180c85577daab044759aa -->

and next.

Thank you!

[Girgias]

The EN-Revision comment tag is the only one you really need.
This value is used to compared to the current git hash of the file in doc-en if they are the same then the file is considered up to date.

If not, a diff of the file will be generated and shown on https://doc.php.net/revcheck.php?p=filesummary&lang=es

[Marqitos]

Thank you!

I will try to dedicate some time to it, and make sure that the documentation in Spanish is not so outdated, since it still talks about PHP 5.