arrow

GH-41673: [Format][Docs] Add arrow format introductory page (#41593)

### Rationale for this change

The documentation for Arrow Format could be improved:

- all types are not listed
- all layouts are not explained

### What changes are included in this PR?

This PR includes:

- motivation behind the columnar format
- different physical layouts explained together with diagrams of example type in comparison to the physical layout
- Arrow terminology
- Extension types and sharing of Arrow data

in a separate "introduction" page with no technical details. Specifications index page is also restructured to include captions and make the left sidebar menu better organised.

**Note:** a table with all types listed together with their physical layout will be added in a separate PR to existing _Columnar.rst_ page: https://github.com/apache/arrow/issues/14752

### Are these changes tested?

No, this is a docs change.

### Are there any user-facing changes?

No.

* GitHub Issue: #41673

Lead-authored-by: AlenkaF <frim.alenka@gmail.com>
Co-authored-by: Alenka Frim <AlenkaF@users.noreply.github.com>
Co-authored-by: Alenka Frim <alenkafrim@Alenkas-MacBook-Pro.local>
Co-authored-by: Raúl Cumplido <raulcumplido@gmail.com>
Co-authored-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>
Co-authored-by: Felipe Oliveira Carvalho <felipekde@gmail.com>
Co-authored-by: Dewey Dunnington <dewey@dunnington.ca>
Co-authored-by: Bryce Mecum <petridish@gmail.com>
Signed-off-by: AlenkaF <frim.alenka@gmail.com>