Docs: align homepage reading order with chapter navigation

Documentation

The documentation homepage presents sections in two columns, but its markup groups entries by column. Consequently, the order when the layout collapses to one column differs from the order obtained by reading the desktop layout row by row.

The root toctree in Doc/contents.rst also uses a different chapter order, making Previous/Next navigation inconsistent with the homepage.

Order of chapters, standard viewports

Order of chapters, narrow viewports

Next topic after last tutorial topic is Python setup and usage (installing/index.rst), which is nowhere near in chapters list

We should establish a consistent sequence across:

• The desktop homepage, read row by row.
• The single-column homepage on smaller screens.
• Previous/Next chapter navigation.

The proposed change is to use a wrapping flex layout in Doc/tools/templates/indexcontent.html, arrange the entries in the intended reading order, and update Doc/contents.rst to match.

Linked PRs

• gh-157528
• gh-157674