Docs: Clarify config file

[peteruithoven]

Feature request

What problem does this feature solve?

Currently when you don't read all the documentation, pages like Writing a theme > Directory structure it can be a bit hard to figure out what is meant with "your theme configuration", what file this refers to. (It's the theme entry / config file, commonly at .vuepress/theme/index.js).
This is especially noticeable when you look at Theme inheritance. And, even worse, the Theme Configuration hardly mentions the default file name and path.

In the case of the Using a theme it's also quite unclear which file this is targeting. (In this case it's the main config file, commonly at .vuepress/config.js).

Proposed change

Let's make sure we mention .vuepress/theme/index.js, the default entry / config file of the theme, more often throughout the documention. The Default Theme Config is a good example, it frequently adds the file path to the config examples, like:

// .vuepress/config.js
module.exports = {
  themeConfig: {
    logo: '/assets/img/logo.png',
  }
}

(I realize that the first tip on Theme Configuration tries to clarify the difference, but it also doesn't mention the file names.)
(Theme Configuration > globalLayout is one of the few places where the file name is added)

I'm not sure if .vuepress/theme/index.js or themePath/index.js is more appropriate & understandable.