feat(cli): support custom srcDir

[Barbapapazes]

Description

Hello 👋,

This PR introduces a new scaffolding parameter to ask the developer where Markdown files should be stored. This is related to the srcDir parameter in the vitepress.config.js file.

For blogs, portfolios, and marketing sites, or any sites that use VitePress as the main project (without a docs folder), having a src folder is more intuitive and I think a better architecture than having every Markdown files at the root of the project. The file .vitepress/config.{js,ts} can stay at the root of the project.

The architecture of the project will look like this:

.
├── src
|   ├── file.md
|   └── file2.md
└── .vitepress
    └── config.{js,ts}

Linked Issues

Additional Context

I'm building a tutorial on how to create a blog with VitePress and Vue.js from scratch. I used a lot VitePress for documentation, and it was fine. But for blogs, portfolios, and marketing sites, as mentioned in the VitePress documentation, I think the CLI needs some adjustments to make it easier to use and less documentation's oriented.

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