[docs] Fix guides links to point to next

[aamirafridi]

• I have followed (at least) the PR section of the contributing guide.

PR to fix this bug

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[cypress]

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Test summary

78 • 0 • 3 • 0

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Run details

Project	material-ui-pickers
Status	Passed
Commit	e862318
Started	Jul 29, 2020 4:28 PM
Ended	Jul 29, 2020 4:30 PM
Duration	01:43 💡
OS	Linux Debian - 10.0
Browser	Chrome 80

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[oliviertassinari]

What about the other cases where the URL is wrong? For instance:

https://github.com/mui-org/material-ui-pickers/blob/df3010bee046034178b5859ce4fd81755cb1d5e4/lib/src/views/Calendar/Calendar.tsx#L24

Also, to be noted that in the main repository, we remove the URL prefix to have the link work no matter the URL it's live on. This comes with one tradeoff, it doesn't work inside IntelliSense.

I think that we should remove the prefixes to prepare the migration in the main repo.