feat(v2): add isCloseable property for theme-classic announcement bar

[Simek]

Motivation

User not always should be able to close the announcement bar. The decision should be on the docs creator side.

This PR adds isCloseable property to the theme-classic announcement bar options, which defaults to true, to retain the previous behavior.

The documentation has been also updated within this PR.

I'm open to rename the property, this was the first, good idea which came to my mind. 🙂

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

Change has been tested locally with Docusaurus V2 website.

Related PRs

• WIP: Migrate from docusaurus v1 to v2 react-native-website#2139

[docusaurus-bot]

Deploy preview for docusaurus-2 ready!

Built with commit eaea10e

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[Simek]

I have added in the second commit fix for the non-cloaseable announcement bar style.

Unfortunately in the way which announcement bar structure and styling is implemented margin-right: 55px is not working "as intend" on the resolution above 1024px due to width: 100% style of announcementBarContent.

To reserve the place for the close button on the right side this can be changed to padding-right on larger resolutions or the general announcementBarContent style can be adjusted. I can address this in the separate PR.

[slorber]

Thanks.

I don't see any issue with both true/false value, that seems fine

[slorber]

Released in alpha 63 🎊 please spread the word 🙏 and tell me if this works fine for you
https://twitter.com/docusaurus/status/1301562737693138944
https://github.com/facebook/docusaurus/releases/tag/v2.0.0-alpha.63