docs: hosted OpenAI-compat get_url must be full /v1/chat/completions

The custom providers example uses get_url returning a full chat URL. Two details are easy to get wrong on a hosted OpenAI-compatible Chat Completions host:

1. get_url must return the full …/v1/chat/completions URL. CopilotChat does not append that path.
2. Reuse copilot.prepare_input / prepare_output. Do not point embeddings at this host.

PZERO is a live OpenAI-compatible host. Catalog: GET https://api.pzero.studio/v1/models. Default text id: deepseek-v4-flash. Prepaid Bearer key (pzero_…).

{
  providers = {
    pzero = {
      get_url = function()
        return 'https://api.pzero.studio/v1/chat/completions'
      end,
      get_headers = function()
        return { ['Authorization'] = 'Bearer ' .. vim.env.PZERO_API_KEY }
      end,
      get_models = function()
        return { { id = 'deepseek-v4-flash', name = 'deepseek-v4-flash' } }
      end,
      prepare_input = require('CopilotChat.config.providers').copilot.prepare_input,
      prepare_output = require('CopilotChat.config.providers').copilot.prepare_output,
    },
  },
}

export PZERO_API_KEY="pzero_…"

Do not return https://api.pzero.studio/v1 from get_url (missing chat path). Do not return https://api.openai.com/... (that is OpenAI, not this host).

No first-party plugin asked. If a named vendor subsection is too much, the gotchas above are the docs gap; the block is a worked example for searchers.

Start with the custom providers documentation linked in the issue and inspect the existing provider example. Update it to show a full /v1/chat/completions URL, the copilot prepare_input and prepare_output helpers, and the relevant PZERO setup and gotchas. Done means the example is discoverable and no longer suggests the incomplete or incorrect URLs.