[IMP] accounting: move in-odoo to top-level TOC page + imp structure

This commit is part of a larger work to improve the documentation's
structure. The toctree structure used to add levels to create categories
and make the structure visually more understandable.
With the new version of the documentation, plenty of these categories
are now redundant.

This commit improves the structure of the "Accounting" app:
- it moves the doc "in-odoo" in the top "app" level and its content is
  included in "accounting.rst" (the content will later be improved)

  - adds new intro
  - changes external links to mementos to correct doc references

- it renames the category accounting/overview to
  accounting/getting_started
- it renames the category accounting/overview/getting_started to
  accounting/getting_started/initial_configuration
- it moves "memento" one level above, from
  accounting/overview/main_concepts to the renamed
  accounting/getting_started as it is the last doc remaining in this
  category
- it renames the app "Invoicing and Accounting" to
  "Accounting and Invoicing" since the focus should be on "Accounting"

task-2535899

content/applications/finance/accounting/reporting/declarations/tax_returns.rst

@@ -37,7 +37,7 @@ and go to the **Tax Return Periodicity** section.
 
 .. note::
    This is usually configured during the :doc:`app's initial set up
-   <../../overview/getting_started/setup>`.
+   <../../getting_started/initial_configuration/setup>`.
 
 Tax Grids
 ---------
@@ -121,5 +121,5 @@ with the amount you have to pay or be refunded.
 
 .. seealso::
    * :doc:`../../taxation/taxes/taxes`
-   * :doc:`../../overview/getting_started/setup`
+   * :doc:`../../getting_started/initial_configuration/setup`
    * :doc:`../../fiscal_localizations/overview/fiscal_localization_packages`