Improve build process docs

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Please improve the documentation pages regarding building and publishing of multiple components.
https://lit.dev/docs/tools/publishing/

This current example

```json
{
"type": "module",
"main": "my-element.js",
"module": "my-element.js"
}
```

is very simplistic and doesn't guide a beginning developer in building several components out of a project in one go.

##### Example:
I have a repository containing a whole library of components (say: `my-button`, `my-item-skeleton`, `my-tabs`) and I am ready to give them a go in my target environment/framework. My questions would be:

1. How do I publish **all of them** in one go? (My expectation is that I will get a bunch of individual files, one per component).
2. How do I have control over what I build?
3. I would like to skip the `npm publish` step, as my pipeline will probably be held internally.
4. Do I need to write/export any loaders?
5. How do I export .d.ts files?

Since I am beginner to LitElement, I will most likely have to do a lot of repetitive tasks before a steady process with good DX settles down. That's why I expect this process to be as much automated as possible, without manual fiddling with the package.json contents.

Start with the publishing documentation at https://lit.dev/docs/tools/publishing/ and review the current package.json example. Expand the page to address publishing multiple components, controlling builds, skipping npm publish, loaders, and .d.ts output; done means a beginning developer can follow an automated process without manually editing package.json.

Written by the indexing model from the issue text.