[Doc] Add environment variable in flow.dag.yaml reference doc

[D-W-]

Description

Please add an informative description that covers that changes made by the pull request and link all relevant issues.
This pull request primarily includes changes to the flow-yaml-schema-reference.md file, which is part of the documentation for the YAML schema. The changes mainly involve modifications to the formatting of the tables in the document. The most significant changes include the adjustment of the column width in several tables and the addition of a new field, environment_variables, in the nodes section.

Formatting changes:

• docs/reference/flow-yaml-schema-reference.md: Adjusted the column width in the tables under the YAML syntax, Flow input, Nodes, and Node variants sections. These changes help to improve the readability of the tables. [1] [2] [3] [4]

Addition of new field:

• docs/reference/flow-yaml-schema-reference.md: Added a new field, environment_variables, to the nodes section. This field allows users to set environment variables by specifying a property path and value.

All Promptflow Contribution checklist:

• The pull request does not introduce [breaking changes].
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.
• Create an issue and link to the pull request to get dedicated review from promptflow team. Learn more: suggested workflow.

General Guidelines and Best Practices

• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message. This means that previously merged commits do not appear in the history of the PR. For more information on cleaning up the commits in your PR, see this page.

Testing Guidelines

• Pull request includes test coverage for the included changes.