docs: clarify custom plugin load ordering

[TyceHerrman]

Summary

• clarify that the custom-plugin convenience loader's file order is unspecified
• recommend keeping dependent plugins in the same file
• document explicit ordered requires for intentionally separate modules
• place the same warning beside the loader implementation

The loader itself remains unchanged.

Closes #2038

Testing

• stylua --check .
• git diff --check

[Christoph-Raab]

Maybe out of scope for the PR, but why doesn't the repo recommand using the official after/plugin way? That would be more nvim native and would save a bit of complexity with the custom init.lua in lua/custom/plugins.

[TyceHerrman]

that could be a better approach, but it's a bigger change. files under after/plugin/ run in a later startup phase. During init.lua, vim.pack.add() defaults to load = false, and after init.lua it defaults to load = true. Moving the current vim.pack.add() and setup() calls into after/plugin/ would change when plugin scripts are loaded. This PR documents how the existing loader works, but Kickstart adopting after/plugin/ as its recommended extension layout would supersede it. so could be closed if that's the direction to go now, but if that needs further deliberation, could merge this PR as is imho