docs: Add blog to website and introductory content

[amol-]

Adds a blog to share news and releases on the Substrait website,
and improves structure of the website to provide a more obvious
getting started path for people interested in Substrait

[github-actions]

ACTION NEEDED

Substrait follows the Conventional Commits
specification for
release automation.

The PR title and description are used as the merge commit message. Please update your PR title and description to match the specification.

[amol-]

Opening as a draft, needs a first example article before the Blog can be published.

A first contact with Substrait isn't super welcoming or obvious for a new user interested in trying it,
and it's hard to "promote" substrait due to the lack of a place where people can keep up to date with improvements, changes and evolutions of the protocol and its implementations.

This PR tries to make a bit more obvious a getting started experience for new users and adds a blog where we can share new releases of libraries and protocol.