Document how to use namespaces in project code

[chillu]

Acceptance Criteria

As a developer:

• I know how to namespace my project classes
• I know how to correctly configure autoloading
• I'm pointed to this information from early-stage introductions (where Page.php is explained in docs and lessons)

Notes

• Should include documentation for the upcoming pull request for controller_name
• There's known workarounds for Page.php namespacing

[maxime-rainville]

There's a bit of that in the upgrade guide:

• Manually namespacing your codebase
• Enable PSR-4 auto-loading in your composer.json file

Maybe some of the details from the upgrading guide need to be moved to this updated doc.

[GuySartorelli]

Pretty much all examples in the docs omit namespaces for the classes being defined. Perhaps we should have an action at to just update the examples so that any time a class is defined, it is given a namespace - and whenever a class is used in an example, a use statement is included.

[GuySartorelli]

This will be handled in #67