Fix: Add aliases for old /docs/contribute/ URLs

[BoHristova]

How to categorize this PR?

/kind bug

What this PR does / why we need it:
The Contribute section was moved from /docs/contribute/ to /contribute/ in #956. The current PR adds aliases to all affected pages so that existing external links (e.g. from other Gardener repositories) continue to work.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

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Important

Review skipped

Review was skipped due to path filters

⛔ Files ignored due to path filters (16)

• website/contribute/contribution-process/contributing-bigger-changes.md is excluded by !website/**
• website/contribute/contribution-process/contributor-guide.md is excluded by !website/**
• website/contribute/contribution-process/roles.md is excluded by !website/**
• website/contribute/documentation/_index.md is excluded by !website/**
• website/contribute/documentation/adding-existing-documentation.md is excluded by !website/**
• website/contribute/documentation/blog-tags.md is excluded by !website/**
• website/contribute/documentation/formatting-guide.md is excluded by !website/**
• website/contribute/documentation/images.md is excluded by !website/**
• website/contribute/documentation/markup.md is excluded by !website/**
• website/contribute/documentation/organization.md is excluded by !website/**
• website/contribute/documentation/pr-guidelines.md is excluded by !website/**
• website/contribute/documentation/shortcodes.md is excluded by !website/**
• website/contribute/documentation/style-guide/_index.md is excluded by !website/**
• website/contribute/documentation/style-guide/concept_template.md is excluded by !website/**
• website/contribute/documentation/style-guide/reference_template.md is excluded by !website/**
• website/contribute/documentation/style-guide/task_template.md is excluded by !website/**

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[klocke-io]

/lgtm

[gardener-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: klocke-io

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[gardener-prow]

LGTM label has been added.

Details

Git tree hash: 6cbf25a6b509fa01a15c85fcac46c87dfe6b53c6