docs: Reduce excessive comments in code examples

[kampalex]

Fixes

Fixes #981

As mentioned, a lot of excessive comments are in example files and docs. I've reduced comments as much as possible.
Also:

• Examples are made executable by loading existing file sendgrid-php.php in project root, instead of requiring an adjustment in the file to adjust path to vendor/autoload.php - which may not exists if composer install isn't executed after pulling library from Git repository.
• Link classes (please report if unwanted!)
• Fixed typos
• Removed invisible whitespace in example files (removed by PHP CS Fixer I think)
• Removed PHP close tag in sendgrid-php.php

Checklist

• I acknowledge that all my contributions will be made under the project's license
• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the Contribution Guidelines and my PR follows them
• I have titled the PR appropriately
• I have updated my branch with the master branch
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation about the functionality in the appropriate .md file
• I have added inline documentation to the code I modified

[thinkingserious]

First, a bit of context. We had a period where were getting a steady stream of issues at our support channel with confusion on installation, which virtually went away with this updated content. So I'm nervous about removing it entirely, but I do agree it's still verbose and likely not necessary for the majority of use cases.

Above this line, I think we should still include:
require 'vendor/autoload.php'; // If you're using Composer (recommended)

Then, something like:
// Comment out the above line and uncomment the next line if you're not using a dependency loader (such as Composer)

And for this part:

// download sendgrid-php.zip from the latest release here,	
// replacing <PATH TO> with the path to the sendgrid-php.php file,	
// which is included in the download:	
// https://github.com/sendgrid/sendgrid-php/releases

we can move those instructions to https://github.com/sendgrid/sendgrid-php/blob/master/README.md#install-package where we can then link from here.

[childish-sambino]

I went ahead and made the changes.

[thinkingserious]

👍