docs: added intro and install instructions

[AlbertoCentonze]

What I did

Improving documentation

How I did it

Adding the following sections:

• [x] introduction explaining why the need for this framework + integrate what is already in the readme
• [x] install instructions (pip, poetry...), how to use the latest version from git.
• [ ] examples of cool things that are possible with vyper (taken from X posts and gh README)
• [ ] examples of boa as a testing framework
• [ ] a section dedicated to people coming from foundry

Minor mentions to be added

• [x] native import syntax
• [x] coverage doesn't work in fast mode
• [x] fast mode
• [ ] lookup_alias
• [x] jupyter notebook magic stuff
• [ ] fixture scope to avoid reloading
• [ ] xdist autouse

Structural changes:

• [x] merge the coverage section in the testing one (the two are short and related).

Other issues:

• [ ] #40
• [ ] #41
• [ ] #42
• [ ] #43
• [ ] #44
• [ ] #152

Description for the changelog

docs: more context on boa usage and examples

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