feat: add support for custom contribution types

[rogerHuntGauntlet]

What: Add support for custom contribution types in the All Contributors specification

This PR adds support for custom contribution types, allowing projects to define their own contribution categories with associated emojis in their .all-contributorsrc file.

Why:

• Projects often have unique ways of contributing that don't fit into the standard categories
• Custom contribution types allow projects to better recognize and categorize these unique contributions
• Provides more flexibility while maintaining the structured approach of the All Contributors specification

How:

1. Added customContributions configuration field in .all-contributorsrc:

   {
     "customContributions": {
       "mentoring": "👨‍🏫",
       "funding": "💵",
       "translation": "🌍"
     }
   }
   

2. Updated core specification documentation to include custom types
3. Enhanced bot and CLI documentation with examples and usage instructions
4. Added notes about current CLI limitations

Checklist:

• Documentation
  • Updated specification.md
  • Updated bot configuration docs
  • Updated bot usage docs
  • Updated emoji key docs
  • Updated CLI usage docs
• Ready to be merged
• Added myself to contributors table

Detailed Changes

Feature Overview

• Support for custom contribution types in the All Contributors bot
• Ability to define custom types with associated emojis in .all-contributorsrc
• Full documentation and examples for both bot and CLI usage

Changes Made

• Added customContributions configuration field in .all-contributorsrc
• Updated specification to include custom contribution types
• Added documentation for custom types in bot configuration
• Added usage examples in bot documentation
• Updated emoji key documentation
• Documented CLI limitations with multiple custom types

Testing Done

• Successfully tested adding a contributor with custom type "mentoring"
• Verified correct updates in both .all-contributorsrc and README.md
• Tested using the CLI locally
• Documented current CLI limitation with multiple custom types

Notes

• The CLI currently has a limitation where only one custom contribution type can be added at a time
• Multiple custom contributions need to be added one at a time or using the bot instead