docs: Add a repo-level README (#169)

* doc: Add a repo-level README

* doc: Improve comparison sections with markdown tables.

* doc: Add note for upcoming feature of default toolset.

* doc: Improve sync/async section

* doc: Improve links in sync/async section

* doc: Simplify sync/async section table.

* doc: Improve support section

* doc: Add load and invocations for toolbox langchain as well

* doc: Fix TOC

* doc: Improve README

* doc: Add note for the upcoming default toolset feature.

* doc: Improve README

* doc: Improve usage comparison table.

* docs: Update repo-level README to only focus on which package to use.

Author: anubhav756