docs: add execution costs to Clarity function reference

[unclekaldoteth]

Summary

This PR adds execution cost information to all Clarity functions in the reference documentation, addressing Issue #1671.

Changes

1. Added Cost Legend Section

Added a new "Understanding Clarity Function Costs" section explaining:

• Cost dimensions: Runtime, Read Count, Read Length, Write Count, Write Length
• Cost types: Static, Linear, NLogN
• Cost notation: How to interpret formulas like 11n + 125

2. Added Cost Info to 60+ Functions

Each function now includes a **cost:** field with:

• Runtime cost formula
• I/O costs where applicable (reads/writes)
• Clear explanation of what n represents

Functions Updated

Arithmetic, Comparison, Boolean/Bitwise, Control Flow, Sequences, Tuples, Options/Responses, Hashing, Crypto, STX/FT/NFT operations, Type Conversion, and more.

Cost Data Source

• stackslib/src/chainstate/stacks/boot/costs-4.clar
• stackslib/src/chainstate/stacks/boot/costs_3.rs
• Version: Clarity 4 (Nakamoto upgrade)

Closes #1671

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