Add "Common Patterns" section to Textbox docs

[yuzegong746-cmd]

Summary

Add a "Common Patterns" section to the gr.Textbox component docs page with two practical recipes.

What's added

Multi-line text input — gr.Textbox defaults to a single line. Shows how to use lines and max_lines for longer text input such as paragraphs or code.

Password and sensitive input — Shows how to use type="password" to mask characters while passing plain text to the function.

Approach

Same pattern as #12755 (Row docs), #12922 (Image docs), and #12923 (Slider docs) — custom markdown section in the .svx file, no docstring changes.

Closes #12748