✨ Search for variants by folder name if no variants by file name found

[YuriiMotov]

This PR is to handle situations when we have versions in different folders (version code is in folder name, not in file name).

In case there is no version found for file in current folder, it will attempt to find versions by folders.
In the example above, for {* docs_src/settings/app02_an/main.py *} it will find:

• Python 3.8+
• Python 3.8+ Annotated

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This implementation only handles variants in parent directory of the file. We may want to support several levels.
For example:

{* docs_src/settings/app02_an/routers/users.py *}

The example above is not currently supported (variants will not be found).

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One drawback of this implementation is that if we have several versions of one app, but one of files is the same in all versions, this file will still be displayed with versions (all of them will be equal).

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To overcome both of issues explained above we can add a config option to specify the level of folder where to search for variants.

For example, to search for variants in app02** folders:

{* docs_src/settings/app02_an/routers/users.py variants_lvl[2] *}

Or, to disable searching for variants in folders (if we don't want to show variants when all of them are equal):

{* docs_src/settings/app02_an/routers/users.py variants_lvl[0] *}

[tiangolo]

Excellent! Impeccable job! 🙌 🚀