[Docs] Documentation and CI

[yashnator]

This pull request adds the documentation generated with documenter.jl, pull request templates and deployment workflow for documentation.

A preview of the documentation can be found at RootIO.jl site from my repo.

To-do:

• [x] Add workflow
• [x] Create pages for introduction, methods & types, getting started and example
• [x] Create examples for I/O of primitive types
• [x] Create examples for I/O of vectors and C-array
• [x] Add pull request template

Checklist before requesting a review:

• [x] Tested on GitHub pages

[grasph]

I've reviewed and improved the documentation. In particular,

• clarified description of supported types. Original documentation was claiming support of both primitive types, but this is in different perspectives: primitive type support was refereeing to entries of individual tree branches/columns, while struct support was referring to mapping a composite type to a tree. In this new version type support always refer to the branch.
• improved the introduction.
• removed the emphasis on using an instance the struct instead of the type it when specifying the row type in the TTree constructor: use the type in the examples, change the argument name from data to rowtype. It is a corner feature included for convenience, but it can confuse the user if it is presented as the canonical usage.
• when the column data are passed to the constructor, as a Table.jl table or as individual vectors, they must be inserted into the tree (*). The behaviour differs from the single instance of a struct mentioned above.
• reorganized the examples.md page and change the examples.

Correction of the code to include the behavior (*) and to fix different limitations seen when testing the old and new examples is included in PR #5.