docs: Add section about configuration override

[cyrilf]

Summary

Add more information about how to override the parent theme default configuration when extending a theme.

If this get accepted, the Chinese documentation still needs to be updated accordingly.

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Other, please describe:

If changing the UI of default theme, please provide the before/after screenshot:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

You have tested in the following browsers: (Providing a detailed version will be better.)

• Chrome
• Firefox
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• IE

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature
• Related documents have been updated
• Related tests have been updated

To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

Related to #2296

[Mister-Hope]

By the way, I can help with the Chinese parts. Just call me if you want to merge it. (To core team)

[ulivz]

I think there is some noise here, you're overriding user's themeConfig instead of config from parent theme.

So this may should not be unsuited to document here.