feat: :sparkles: Add `fetch_roles_member_counts` method to `Guild` and corresponding HTTP route

[Paillat-dev]

Summary

depends on https://github.com/discord/discord-api-docs/pull/8025

Ref: https://discord.com/channels/881207955029110855/1411737823635312693/1446851187625164827

Information

• [ ] 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, typehinting, examples, ...).

Checklist

• [ ] I have searched the open pull requests for duplicates.
• [x] If code changes were made then they have been tested.
  • [x] I have updated the documentation to reflect the changes.
• [ ] If type: ignore comments were used, a comment is also left explaining why.
• [x] I have updated the changelog to include these changes.

[pycord-app[bot]]

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git fetch origin pull/3020/head:pr-3020
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pip install git+https://github.com/Pycord-Development/pycord@refs/pull/3020/head