[decimal] Implement arctan() and pi() for BigDecimal

[forfudan]

BigDecimal Enhancements:

• Introduced unsafe methods for constructing BigDecimal from raw components (from_raw_components), with detailed documentation explaining their usage and risks.
• Added new mathematical operations to BigDecimal, including arctan, power, and constants like pi and e. These additions expand the library's capabilities for advanced calculations. [1] [2]
• Improved methods for printing BigDecimal representations, including print_representation_as_components, which provides a detailed breakdown of the number's components. [1] [2]

Mathematical Constants and Functions:

• Added a precomputed constant PI_1024 for π to 1024 digits of precision and implemented a function pi(precision) to calculate π using Machin's formula for higher precision.
• Updated the exp_taylor_series function in exponential.mojo to clarify the precision handling for natural exponential calculations.

Documentation Enhancements:

• Added a detailed quick-start guide in README.md to showcase the decimojo.prelude module, including examples of basic arithmetic, mathematical functions, and internal representation of numbers. [1] [2]
• Created a new example file docs/examples_on_bdec.mojo to demonstrate the usage of BigDecimal for arithmetic, mathematical, and internal representation operations.