Adding UI and UX changes and moving the content to the new documentation engine

[iamrajiv]

• [x] Implementing UI and UX changes in the documentation engine like creating components, templates, standalone pages, etc.

  Created the new components, templates, standalone pages, etc for the documentation engine. I had refactored the default components and templates of the Docusaurus site which were added when the Docusaurus site was initialized.

  Above task is done and I have pushed the changes directly to the repository. Commit can be found here https://github.com/keptn-sandbox/new-keptn-docs-engine/commit/b749bbdfebfeaa41c789cd4c91ac29032ac1bbfa.

  After getting feedback from mentors on the new documentation engine, I have refactored the documentation engine. PR can be found here https://github.com/keptn-sandbox/new-keptn-docs-engine/pull/9.

• [x] Finalizing the things that need to be copied from the current documentation engine to the new documentation engine with proper structure.

• [x] Copying content from the current documentation engine to the new documentation engine which has been finalized.

  As of now not all contents are copied from the current documentation engine to the new documentation engine instead, some documents are copied to the new documentation engine.

• [x] Adding documentation search functionality.

As of now I have added local search support using a docusaurus-lunr-search local search plugin but in the future, we can update this to Algolia DocSearch.