Add documentation how to use the YAML merge key (example supplied)

[silverham]

YAML merge works in lando.yml file.
We should document this on lando.dev.

Example of merge working in lando/yml file:

Merged version

name: my-drupal9-project
recipe: drupal9
x-php-version: &php-version '8.1'
x- nested-config-wrapper: &nested-config-wrapper
  config:
    php: lando-config/php.ini
config:
  php: *php-version
  << : *nested-config-wrapper

Equivalent

name: my-drupal9-project
recipe: drupal9
config:
  php: '8.1'
  config:
    php: lando-config/php.ini

More information
Merge Key Language-Independent Type for YAML™ Version 1.1
Lando Drupal Drupal Configuration
Sample online YML parser - codebeautify
Sample online YML parser - appspot