docs(learncss): Add Transitions guide

[camden]

Fixes #5727

Changes proposed in this pull request:

• add cambickel to web.dev contributors
• add Transitions guide to LearnCSS
• add Transitions page to LearnCSS Table of Contents

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[chrome-devrel-review-bot]

Hello! This is an automated review by our custom reviewbot. It updates automatically when code or GitHub comments in this pull request are created or updated.

Requested changes

If there are any common problems with the content files you created or modified, they will be listed here.

src/site/content/en/learn/css/transitions/index.md

• Please put a blank line before and after each section heading. Some Markdown parsers don't detect the text as a section heading unless there is blank space before and after it. Affected lines: 31, 34, 53, 64, 81, 100, 116, 125, 136, 148, 159, 170, 186, 211, 238

[chrome-devrel-review-bot]

Hello! This is an automated review by our custom reviewbot. It updates automatically when code or GitHub comments in this pull request are created or updated.

Requested changes

If there are any common problems with the content files you created or modified, they will be listed here.

src/site/content/en/learn/css/transitions/index.md

• Please put a blank line before and after each section heading. Some Markdown parsers don't detect the text as a section heading unless there is blank space before and after it. Affected lines: 31, 34, 53, 64, 81, 100, 116, 125, 136, 148, 159, 170, 186, 211, 238

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[rachelandrew]

Mostly URLs, removing en-US from the MDN links so the reader gets their localization, and removing web.dev from internal links plus a few copy edits.

[argyleink]

looks rad! looks like you had fun!

[una]

Great module! I just added a few nits :) Thanks @camden

[camden]

Updated a handful of demos to improve color contrast (and color variety) and consistency, and just to make them look a little better.

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[camden]

@googlebot I consent.

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