Mismatch between repo path and collection folder when deploying from a monorepo

I think I'm looking for some concept like a "root dir".

In my setup, I have a nuxt app which serves decap and render's its content. The thing is, the app is in a subdirectory in my repository and the entire thing is a monorepo. It looks something like

```
/
└── nuxt-app/
└── content/
├── blog
├── handbook
└── pages
```

In my config, I have these collections to match

```yaml
backend:
name: github

collections:
- name: handbook
label: "Handbook articles"
label_singular: Handbook article
identifier_field: id
folder: "content/handbook"
extension: md
widget: "list"
create: true
i18n: true
summary: "{{title}}"
view_groups:
- label: Section
field: section
path: '{{section}}/{{slug}}'
fields: []

- name: pages
label: "Pages"
folder: "content/pages"
extension: md
widget: "list"
create: false
i18n: true
fields: []

- name: blog
label: Blog
label_singular: Blog Post
folder: "content/blog"
extension: md
widget: "list"
create: true
i18n: true
fields: []
```

See that each collection's folder is `content/*`, which works when I add `local_backend: true`. But when I run the deployed app, I assume decap is looking for `content/blog`, etc in the root of the repository.

I see a number of possible solutions but none that exist

1. Add an option to `local_backend`

```yaml
local_backend:
root: nuxt-app

collections:
- name: blog:
folder: nuxt-app/content/blog
```

This way, the `folder` must match the repo but `local_backend` removes it when serving local. Not super fond of this, as it appear a little unintuitive

2. Add an option to `github` backend

```yaml
backend:
name: github
rootDir: nuxt-app

collections:
- name: blog:
folder: content/blog
```

A little better, keeps the relative path for `folder` options but the additional `rootDir` property of GH backend tells it look for content in the right subdirectory.