Add github-pages & update landing page docs

[lucianojoublanc-da]

This adds a root docs/ folder where documentation is now generated, for #45 . Generating this automatically with CI is more complicated and I couldn't get this working after an hour of tinkering. The downside of this is that we need to commit changes to the docs/ folder manually for the time being, after building docs locally (using make build, or `cd python && make docs). Would be nice to switch to CI doc builds in the future.

To enable the github pages docs, you'll need to enable them in github settings, toward the bottom of the page. The docs will then be hosted online at https://digital-asset.github.io/dazl-client

Also note that there appears to be something wrong with the CSS styles. Would appreciate if somebody with more knowledge of CSS/Sphinx could check that out.

I updated the README/index files to reflect that the build system is now Poetry, and also added a note that the library can be installed using pip, for the casual user.

[lucianojoublanc-da]

You can preview the docs at https://lucianojoublanc-da.github.io/dazl-client

[da-tanabe]

Closing this fork-based PR in favor of a branch-based one (#103).