feat(plugin-docsearch): support indexBase option (close #1223)

[Mister-Hope]

No description provided.

[meteorlxy]

Do you think we need to support different indexBase in different locales?

[Mister-Hope]

Do you think we need to support different indexBase in different locales?

Maybe, I am not sure.

Since our locales is based on file structure and a bit hard to customize (there are hard links in serverl places like homepage frontmatter actions option, navbar options, sidebar options, etc), I am not expecting users to reorganize files with scripts or use permalinks to override them and deal all hard links.

For my users cases, they are not changing locales.

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So, I do think maybe its fine to just provide an extra indexBase, if they are actually changing 2 sites, they may need to consider running another crawler as the firt choice?🤔

[meteorlxy]

OK, let's keep it simple to meet most use cases. I've updated the docs and you could check it again.

[Mister-Hope]

LGTM