Add "String" Section

[danielcristho]

The "String" section is currently marked as Coming Soon. Strings are a fundamental concept in Python and should be introduced early to help learners understand text processing and basic data manipulation.

Scope of Material:

• What is a string
• String literals ('...', "...", """...""")
• concatenation
• indexing & slicin

Introduce string methods like:

1. .lower(), .upper()
2. .strip()
3. .replace()
4. .split()

String formatting:

1. f-strings
2. format()

Goals

• Provide clear intro to workin with strings in Python
• Help understand text manipulation