Docs for matrix expressions

What does this MR do and why?

This MR adds documentation for the new matrix expressions feature. This MR also introduces users to the new concept of CI/CD expressions, which now includes matrix expressions as well as our existing inputs expressions.

Matrix expressions issue: Support dynamic job dependencies in `needs:para... (#423553)

Matrix expressions feature flag rollout issue: [FF] Rollout ci_matrix_expressions gitlab_com_d... (#571105)

Matrix expressions main implementation MR: Add matrix expressions (!205247 - merged)

What's being added

New documentation pages

• doc/ci/yaml/expressions.md - Overview page explaining the expression system and different contexts (inputs and matrix)
  • Review app: https://docs.gitlab.com/upstream-review-mr-ee-205425/ci/yaml/expressions/
• doc/ci/yaml/matrix_expressions.md - Main page for matrix expressions
  • Review app: https://docs.gitlab.com/upstream-review-mr-ee-205425/ci/yaml/matrix_expressions/

Enhanced existing documentation

• doc/ci/yaml/_index.md - Added matrix expressions section
• doc/ci/_index.md - Updated Step 4 to introduce both variables and expressions to new users