[IMP] Taxes: improve tax configuration doc

task-5094417

This commit rewrites the Taxes page. Previously, it came across as a
huge list of tax configuration options. We now structure it as follows:
1. an overview of the tax flow in Odoo;
2. a set-up guide for taxes for new users;
3. an explanation of the most common configuration options.

The options related to the tax computation are split into a separate
Tax Computation page, in which we also improve the following sections:
- Tax computations:
  At the moment, the section for the `Percentage of Price tax included`
  tax computation doesn't distinguish itself enough from the 'Included
  in Price' option which people usually want.
  We also improve the examples.
- Included in Price
- Affects base of subsequent taxes
- Base affected by preceding taxes: this option was not documented, so
  we add a section for it.

Ultimately, we plan to rewrite the Tax Computation page to provide a
better explanation of how the tax computation actually works, and be
more helpful at guiding the user to configure their taxes.

X-original-commit: 64e8e5b

Author: antoine162