Update to the latest Docusaurus version. (2.0.0-beta.15)

[abhigyantrips]

I'm leaving this as a separate issue, since there are some conflicts and breaking changes I'll have to rewrite for. Most changes during the version change will be related to the config. files and the components, So this shouldn't affect the main content workflow.

A few reasons for switching over to the new version, since it isn't necessarily compulsory:

• Auto-collapsing sibling categories, which will clear up the clutter in the sidebar;
• HTML overriding, can be useful in a few places;
• Disabling certain components in dev, which will reduce the payload on the contributor's machine;
• Contextual search being default, will affect Algolia DocSearch indexing.

The breaking changes for which we'll have to refactor:

• All theme hooks have now been moved to @docusaurus/theme-common, and now come under useColorMode. Since we have multiple components depending on this, it'll have to be rewritten.
• The components that have been manually swizzled (named MDXPage and MDXComponents) will have to be reswizzled and refactored with the changes made earlier. This is important since those components underwent quite a few changes.

All of this is covered in greater detail at https://docusaurus.io/changelog/2.0.0-beta.15.

[abhigyantrips]

Note that this is not high priority - since the release is still new, I'd rather wait for bugs to be reported and fixed, than run after updating to the newest version.