Sleak/update spack docs

Update Spack docs for 0.16.1 Preview at https://portal.nersc.gov/cfs/mpccc/sleak/userdocs4/development/build-tools/spack/

Submitters and reviewers please make sure

• internal links are relative and to the .md file
• avoid links named "here"
• alt-text for images is descriptive
• doesn't duplicate existing content
• spell check
• renders correctly
• does not include any identifiable information (real usernames, project names, etc)