Webdoc: Revise intro & installation instructions

From #523 (closed):

• https://bornagainproject.org/ old GUI screenshot

• https://bornagainproject.org/git-main/installation/building/windows/ mentions Qt5

• https://bornagainproject.org/git-main/installation/building/unix/third-party/ mentions Qt5 (#552 (closed), !1595 (merged))

• https://bornagainproject.org/git-main/installation/building/unix/build-and-install/

sudo make install <-- DO NOT DO THIS

installation paths <-- no installation anymore

$ cmake -DCMAKE_INSTALL_PREFIX=<install_dir> ..
$ cmake -DCMAKE_PREFIX_PATH=/opt/local -DCMAKE_INSTALL_PREFIX=<install_dir> ..

Installing the framework <-- no installation anymore

• https://bornagainproject.org/git-main/py/

Installing the Python package <-- mv to Installation page?

Then read on to understand some example scripts line by line, and to discover further functionality <-- The link should refer to https://bornagainproject.org/git-main/py/start/syntax/