Remove out of date statement

The page describes the intended usage of the 'scripts' and 'qt' directories.

Re #24293

Document how to run single Python unittest functions.

Also tidies up this section so there is a single list of packages that
you must install.

- Name changed stresstesting.MantidStressTest -> systemtesting.MantidSystemTest
- Update the dev-docs as well

Re #0

The instrument/IDFs_for_UNIT_TESTING directory was renamed to instrument/unit_testing, as it now contains nexus as well as IDF files.
All the paths in the source were changed accordingly.

The SMALLFAKE_example_geometry.hdf5 was moved from the build ExternalData to instrument/unit_testing.

The LOKI_Definition.hdf5,SANS2D_Definition_Tubes.hdf5,WISH_Definition_10Panels.hdf5 files were moved
from the build ExternalData to instrument/unit_testing.

Update copyright headers in all files

Squashed commit of the following:

* First version of a script to manage copyright statements

re #23468

* Neatened up script, added comments

re #23488

* move script to tools directory

re #23488

* small script changes and a couple of manual file changes

re #23488

* Minor chnage to whitespace detection in regex

re #23488

* Add an excluded directory

re #23488

* remove a repeasted copyright statement in a file

re #23488

* Don't comsume the comment end if it is on the same line

re #23488

* fix error in new copright parsing

re #23488

* remove double copyrifght entry

re #23488

* Improve handling of old copyrights at the start of comments

re #23488

* remove empty comments re #23488

* exclude gsoapgenerated directories

re #23488

* Sort out greedy line matching re #23488

* improve empty comment removal

re #23488

* improve false positives

re #23488

* impressive speedup by limiting regex matching length

re #23488

* remove evil invisible non ascii character

Also upadte the copyright at the same time

re #23488

* resolve multiple copyrights in a single file

re #23488

* resolve an issue with new statement detection

re #23488

* another unprintable unicode character

re #23488

* pep updates and cmake the new copyright fit clang format

re #23488

* update already done new format headers

re #23488

* wrong type of bracket

re #23488

* Update class_maker and friends

re #23488

* Update all copyright statements

re #23488

* clang format re #23488

* flake8 warnings re #23488

* Flake8 warnings re #23488

* Exclude .cmake.in and rb.in files

re #23488

* replace missing line re #23488

* exclude .py.in files as they are flasely recognized as C++

re #23488

* another setp.py.in re #23488

* another .py.in correction re #23488

* Hopefully the last of the .py.in files re #23488

* resolve utf-8 encoding of python files and changed ABINS checksum

re #23488

* updates to unit tests that reference line numbers

re #23488

* remaining unit test files and other fixes

re #23488