Add tests and documentation about how CI "extends" can be excluded

What does this MR do and why?

Related to #36372 (closed)

This MR adds tests and documentation about how CI "extends" can be excluded.

The "extends" keyword works via reverse a reverse deep merge. So, when you want to exclude a key from it, adding "{}" won't work. You need to assign it to "null".

Screenshots or screen recordings

There is a more detailed example in a new test case but here is another one;

Example 1

.base:
  variables:
    VAR1: hello
  script: exit 0

job:
  extends: .base
  variables:
    VAR2: mello

The result is;

job:
  variables:
    VAR1: hello
    VAR2: mello
  script: exit 0

Example 2

.base:
  variables:
    VAR1: hello
  script: exit 0

job:
  extends: .base
  variables: {}

The result is;

job:
  variables:
    VAR1: hello
  script: exit 0

Example 3

.base:
  variables:
    VAR1: hello
  script: exit 0

job:
  extends: .base
  variables: null

The result would be;

job:
  script: exit 0

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