[core] Batch small changes

[oliviertassinari]

• [core] JSDOC -> JSDoc f33ba3f: This seems to be the official name: https://en.wikipedia.org/wiki/JSDoc.
• sort props asc ad69034: a small detail.
• [website] Fix small mistakes in the branding 4c0c751: Fix 404 link, fix design regression with link.
• [website] Fix Link underline aesthetic regressions a0b62ee: In a couple of use cases, the link behavior is implicit and doesn't need the underline to be explicit.
• [docs] Fix demo in dark mode ba0434d: Open https://next.material-ui.com/customization/palette/#default-values and turn on dark mode.
• [core] Sort props asc ab2c468: a small detail.
• [core] Apply @tag convention da29091: a convention we have been using so far.
• [docs] Improve notification color b5308fc: Same as [docs] Improve notification color #26796. It displays funny https://next.material-ui.com/components/app-bar/#app-bar-with-a-primary-search-field.
• [docs] Remove outdated API fa98959: textSecondary -> text.secondary cc @siriwatknp.
• [core] Update import of React to match convention 066eda3: a small detail
• [docs] Apply quotes convention df57b19: a small detail
• [docs] Use the same spacing between the demos 766bd15 4ab4c91: I have noticed that the button demos felt inconsistent. I have updated the set to use 16px for buttons and 8px for icon buttons.
• [docs] Fix header inversion 9d543bf: The headers were no longer in the logical order. I suspect a merge conflict.
• [docs] Dispaly code previews on mobile 5e05cf0: The new behavior:

• [test] Polish RenderCounter description 39cab97: A follow up on [test] Count profiler renders not passive effects #26678 (comment)
• [docs] Apply FIXME 3c82ad2: From https://github.com/mui-org/material-ui/pull/26774/files#r659238100

I have noticed more opportunities, but the others need to be in dedicated PRs.

[mui-pr-bot]

Details of bundle changes (experimental)

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