Create documentation based on markdown

[Siemienik]

Documentation

For each project, the documentation is a significant place where the customers derive knowledge about how to use that tool. That explains why it is worth to care about the experience of using the documentation. For this purpose, this issue was created.

Current

Our current documentation has a repository: https://github.com/handsontable/docs, and it is hosted on https://handsontable.com/docs/. The content is kept in HTML format. The current search is searching only over sidebar labels.
That makes that using and searching the documentation is difficult.

After

We gain nicer looking documentation with a better search engine. Using markdown makes easier documentation maintenance for non-technical personnel. After this issue being resolved, versioning and automated deployment will be simpler.
Creating and testing documentation becomes less time-consuming, and for our clients, we remove difficulties in using and searching the documentation.

Assumptions:

• Move our documentation to this repository, which will help manage it during the development process.
• Convert all documents from the current HTML format to Markdown. That makes it much more manageable and technology independent.
• Improve the graphical layout of our documentation, which makes using it much more convenient.
• Change search engine to a new one with a much better experience.
• Automate the deployment process by using GH Action,
• Ensure that will works with previous documentation versions.
• The new documentation should be placed in this repository in /docs/ directory.