Add 7.8 in version

[rajasekarm]

Updated latest version.

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[nicolo-ribaudo]

Thanks!
We actually have a script in the website/ package which bumps the version and generates the versioned docs: cd website && npm run version.

[existentialism]

FWIW, I would have preferred it if we ran the script instead of landing this... as it's nice to have just the files that changed from version to version instead of every doc getting added to the version-7.8.0 folder (like https://github.com/babel/website/tree/master/website/versioned_docs/version-7.7.0).

[nicolo-ribaudo]

The script copied every file because in #2150 I modified every doc.
I should work on backporting the ### With a configuration file (Recommended) changes and then re-run the script to clean up the 7.8.0 folder.

[existentialism]

Ahh... yeah, not critical, just a nicety.