docs: add Neovim integration guide and fix http link

[sedat4ras]

Description

This PR addresses two documentation improvements:

1. Add Neovim integration guide (fixes #3960)

Neovim has grown significantly in popularity but was completely missing from the editor integrations docs. The Vim section only covers Vim-specific plugins.

The new Neovim section covers three common approaches:

• conform.nvim — the most popular modern formatter plugin for Neovim, with a format-on-save example
• ALE — cross-references the existing Vim section since ALE works with both
• Simple command — no-plugin approach using :!black % with a Lua nvim_create_user_command example

2. Fix http:// link in docs/index.md

The testimonials section linked to http://python-requests.org/ which redirects to https://. Updated to use https:// directly.

Checklist - did you ...

• Implement any code style changes under the --preview style, following the stability policy? — N/A (docs only)
• Add an entry in CHANGES.md if necessary? — Yes, added changelog entry for the Neovim guide
• Add / update tests if necessary? — N/A (docs only)
• Add new / update outdated documentation? — Yes, added Neovim section and fixed http link