Allow README.md to be blog index file name

### Have you read the Contributing Guidelines on issues?

- [X] I have read the [Contributing Guidelines on issues](https://github.com/facebook/docusaurus/blob/main/CONTRIBUTING.md#reporting-new-issues).

### Description

see `packages/docusaurus-plugin-content-blog/src/blogUtils.ts:146`:

```ts
const text = blogSourceRelative.replace(/(?:\/index)?\.mdx?$/, '');
const slug = `/${text}`;
return {date: undefined, text, slug};
```

It's easy to add the support of rendering `index.md`, `readme.md`, etc. to be the index page if there are only one unique file (with these possible names) under a specific folder.

As far as I know, many blog framewroks support both `index.md` and `readme.md` to be rendered as the index page.

### Has this been requested on Canny?

_No response_

### Motivation

The github use the default `readme.md` (or `README.md`, which is case-insensitive) as the default rendered file.

Hence, there are a lot of projects (including blog) with their introduction file naming with `readme.md` rather than `index.md`.

So it would be quite delightful if we docusaurus can support this design.

### API design

I feel it OK with either an extra API field for our users to manually point out what index filename is the best, or just do a litter more work in our blogUtils logic, in order to translate the md file to slug accuarately.

### Have you tried building it?

_No response_

### Self-service

- [X] I'd be willing to contribute this feature to Docusaurus myself.