Center images in sections and add video.js to personalize video controls

[conradolandia]

Pull Request

Pull Request Checklist

• Read and followed this repo's Contributing Guidelines
• Based your PR on the latest version of the master branch
• Checked your writing carefully for correct English spelling, grammar, etc
• Described your changes and the motivation for them below
• Noted what issue(s) this pull request resolves, creating one if needed

Description of Changes

• Add margin: 1rem auto to section img to center images in sections.
• Add video.js to improve video controls UI/UX. This adds new control styles to videos that can be further personalized using CSS. Videos can still be added using sphynxcontrib-video markup in rst, but we will need to remember to add :nocontrols: to every video to avoid duplicated controls.

Issue(s) Resolved

• Images were misaligned.
• Video controls became hidden when the user moved the pointer away from the video.

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[ccordoba12]

Looks good to me, thanks @conradolandia!