Updating README.md to reflect re-organised software pages

This is an update of the main README to point to the new links in atlassoftwaredocs. It also makes some modifications to the text.

tagging @jchapman @emoyse

This merge request affects no known packages. Consult an expert!

This merge request affects 1 file:

• README.md

Could not map the following files to a domain:

• README.md

added main no-domain review-pending-level-1 labels

The system determined that the changset affects text files and cancelled CI integration tests

CI Result SUCCESS (hash 0549dcd2)

	Athena	AthSimulation	AthGeneration	AnalysisBase	AthAnalysis	DetCommon
externals
cmake
make
tests

Full details available on this CI monitor view. Check the JIRA CI status board for known problems
Athena: number of compilation errors 0, warnings 0
AthSimulation: number of compilation errors 0, warnings 0
AthGeneration: number of compilation errors 0, warnings 0
AnalysisBase: number of compilation errors 0, warnings 0
AthAnalysis: number of compilation errors 0, warnings 0
DetCommon: number of compilation errors 0, warnings 0
For experts only: Jenkins output [CI-MERGE-REQUEST-EL9 14623] (remote access info)

The changes are good from my side. Approved from L1.

added review-approved label and removed review-pending-level-1 label

merged

mentioned in commit 6d2d7581