Docs/add contribution guide

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This PR adds a contribution guide to the Espressfi Developer Portal.

Resolves #18.

To view it, do the following on your local machine

• Run

  git clone --recursive ssh://git@gitlab.espressif.cn:27227/kirill.chalov/developer-portal.git
  cd developer-portal/
  git fetch
  git checkout docs/add_contribution_guide

• In the repo, go to content/pages/contribution_guide
• For more convenient view, install Hugo and run hugo server in the project folder

Other things

• For convenience, the Contribute link is also added in the footer navigation bar of the website.

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The review has already been done internally. This PR is ready to be merged.