fix documentation for install into empty directory

**Describe the bug**
The documentation can be confusing about how to install UUV.

**To Reproduce**
Steps to reproduce the behavior:
1. Create an empty directory: `mkdir test-uuv`
2. Go into this directory: `cd test-uuv`
3. Install UUV as stated in the documentation: `npm install --save-dev @uuv/cypress`
4. Compare the content of the directory with the contente expected as stated in the documentation

**Expected behavior**
As stated in [the documentation](https://orange-opensource.github.io/uuv/docs/getting-started/installation):

```bash
uuv/
cypress.config.ts
cypress/
support/
command.ts
.cypress-cucumber-preprocessorrc.json
```

The real content of the directory is:

```bash
node_modules
package.json
package-lock.json
```

The documentation should mention that the install should be made in an existing npm project. In other words, if the directory is empty, you should create a valid package.json file by creating the file manually or by launching a `git init` command.