feat(Colour): factory method for `#RRGGBB` color codes

[Enegg]

Summary

This PR adds a new factory method .from_hex_str to disnake.Colour which accepts strings in the form of "#RRGGBB". This is particularly useful in code editors that provide a color picker when they detect a color code.

Checklist

• [x] If code changes were made, then they have been tested
  • [x] I have updated the documentation to reflect the changes
  • [x] I have formatted the code properly by running uv run nox -s lint
  • [x] I have type-checked the code by running uv run nox -s pyright
• [ ] This PR fixes an issue
• [x] This PR adds something new (e.g. new method or parameters)
• [ ] This PR is a breaking change (e.g. methods or parameters removed/renamed)
• [ ] This PR is not a code change (e.g. documentation, README, ...)

[shiftinv]

ext.commands.ColourConverter also handles hex colors of various formats, it could probably make use of this new method too.

[Enegg]

ext.commands.ColourConverter also handles hex colors of various formats, it could probably make use of this new method too.

I did expect there being a converter for this, but I'm not convinced this simple method is going to be of much use to it.

I can see these two relevant methods: https://github.com/DisnakeDev/disnake/blob/9ebc9deac902989e6d0c96e93f8726aeb14e4af9/disnake/ext/commands/converter.py#L705-L714 https://github.com/DisnakeDev/disnake/blob/9ebc9deac902989e6d0c96e93f8726aeb14e4af9/disnake/ext/commands/converter.py#L738-L757

...but they perform work/validation that goes beyond simply stripping the leading # and parsing as base 16. It also seems to support 3-digit #RGB format.