Update DDEV instructions, so users are not required to have PHP and Composer installed on the host system

[gitressa]

Existing document

• https://docs.localgovdrupal.org/devs/getting-started/
• https://docs.localgovdrupal.org/devs/getting-started/working-with-ddev.html

In what way is the existing documentation unclear or incomplete

Currently, the method used in the Working with DDEV page requires that PHP and Composer are installed on the host machine, but this can lead to version conflicts, and strange behaviour. See also:

• Don't require Lando users to have Composer\PHP on their host system localgov_project#59

What should the documentation say instead?

It would be better to use a method where Composer is only run from DDEV.

See MR:

• Update DDEV instructions, to use Composer in DDEV only #342

Update installation instructions on project page?
Also, the LocalGov Drupal project page could link to https://docs.localgovdrupal.org/devs/getting-started to prevent it from having outdated instructions?

Installing LocalGov Drupal locally

To install LocalGov Drupal locally for testing or development, use the Composer-based project template.
[...]

Maybe better?

Installing LocalGov Drupal locally

To install LocalGov Drupal locally for testing or development, follow the instructions on the Getting started page.