Documentation: Add installation instructions for xmllint which is enabled by cbuild tool when -s

[jkrech]

Installation instructions lists ninja and cmake but not xmllint.

cbuild -s runs xmllint for all generated *.cprj before running cbuildgen to generate CMakeLists.txt files

see existing documentation: https://open-cmsis-pack.github.io/devtools/buildmgr/latest/cbuild_install.html#libxml
see also https://github.com/Open-CMSIS-Pack/gen-pack#xmllint as reference how to obtain/install xmllint.

[ReinhardKeil]

Given the fact that *.cprj is a legacy format, should we remove the -s option from cbuild instead?

[jkrech]

This issue got fixed: Open-CMSIS-Pack/devtools#1261
xmllint is only used in case of cbuild buildcprj --schema command
Please reject if this corner case use case is not important enough to explain the xmllint installation.

[ReinhardKeil]

suggest to reject this issue.

[jkrech]

Given the fact that we don't expect cprj formatted files are used as input for cbuild much, we decided not to extend/clutter the documentation potentially distracting new users.