Docusaurus: Unable to upgrade to newer alphas

[phated]

There are some breaking changes in Docusaurus alpha releases and we aren't able to upgrade to them automatically.

Need to research what is breaking and get us updated.

1. Documentation in docusaurus for the reference: https://github.com/facebook/docusaurus/blob/master/CHANGELOG-2.x.md
2. Issue: If we try to increase the version number, our website breaks
3. Discord: https://discordapp.com/invite/docusaurus

@phated After the review of the reference, it will be helpful if I get more details.
Is there any capture of the screen about the error? If it is possible it will be great to know the procedure which occurs this error.

[phated]

As noted in #129 (comment), we should remove the workaround for the styling on the copy button once we upgrade.

Checked the actual problem and will proceed to debug this issue.

From the https://discord.com/channels/398180168688074762/584803742801723424

Narrow the issue to the docusaurus.config.js file. Fix the 4 files below and 'npm run start' worked.(Pull requested)

1. Plugins: With the explanation as written by Sebastien Lorber in the link isProd seems not valid and this is only using on building production, thus no need. Furthurmore no error after delete isProd
2. Theme color: modified them color config in accordance with docs file in docusaurus. Please confirm pull requests file
3. Navbar links parameter: links has been changed to items so modified, no error after change
4. Footer title is mandatory: Added title in the footer array

With these four modification application starts running. Had issue after running, but this seems local error.

[phated]

Thanks for all your work on this @donghwipark!! Closed by #133