text: Title: Documentation - Clean up the *Reference > Pages* sub-pages to be only about Page models
  Body: ### Pertinent section of the Wagtail docs
  
  > https://docs.wagtail.org/en/stable/reference/pages/index.html
  ![Image](https://github.com/user-attachments/assets/ac9d2dee-c0ab-4f7c-8360-165a9477a122)
  
  
  ### Details
  
  * Within the `Reference` section of our documentation, we have a sub-page called `Pages`. This contains the 'Model reference' and 'Panel types' documentation, however, these are not specific to the Page models.
  * I have found it confusing but have kind of gotten used to it, but from a newcomer perspective it feels odd. Panels are now fully supported across Snippets and Pages, the Model reference page has all models (not just pages).
  * There are a few other changes that would make this 'Reference > Pages' section a bit more consistent and predictable.
  
  #### Proposed changes
  
  1. Move [`Panel types`](https://docs.wagtail.org/en/stable/reference/pages/panels.html) directly under [`Reference`](https://docs.wagtail.org/en/stable/reference/index.html).
  2. Move [`Model Reference`](https://docs.wagtail.org/en/stable/reference/pages/model_reference.html) directly under [`Reference`](https://docs.wagtail.org/en/stable/reference/index.html).
  3. Update the title of [`Model Reference`](https://docs.wagtail.org/en/stable/reference/pages/model_reference.html) to `Model reference` (lower case `r`, as this is the way most page titles are capitalised and `Reference` is not a proper noun.
  4. Split the **tagging** section from [`Recipes`](https://docs.wagtail.org/en/stable/reference/pages/model_recipes.html) out to a new page called Tags to directly under `Advanced topics` called `Tags`.
  5. Fix the incorrect heading level nesting in [`Recipes`](https://docs.wagtail.org/en/stable/reference/pages/model_recipes.html) for the 'adding endpoints with custom route methods, it should be the same level as 'overriding the serve method' not one under it.
  6. Minor update to the text `wagtail.models` to use double backticks so that it appears highlighted as code. See https://github.com/wagtail/wagtail/blob/105338d0d63c5a8adf285d3c077a922f53a15d02/wagtail/models/__init__.py#L2 (we may also want to fix up the other backtick usage in that docstring to be double backticks)  * See https://github.com/wagtail/wagtail/blob/105338d0d63c5a8adf285d3c077a922f53a15d02/wagtail/models/__init__.py#L331C16-L331C31 for an example of the double backticks being used correctly to style as pre/code in the published docs. 
  
  ### Working on this
  
  * Feel free to cover items 1-3 as a stand-alone PR, it may make reviews easier.
  * Anyone can contribute to this. View our [contributing guidelines](https://docs.wagtail.org/en/latest/contributing/index.html), add a comment to the issue once you’re ready to start.
  