Update installation instructions

e.g. update Install.md starting with what Tarun sent in e-mails (for Linux, macOS, and Windows) - do test

changed milestone to %Organisation for the 4th workshop

assigned to @oschmanf

Also from feedback notes #43 (closed) from last workshop:

• opt, add autocompletion: see jupyterlab-lsp installation instructions (mv'ed to #58 (closed))
• python was spec'ed 3.8 resulting in installation of 3.8.0 => should be specified to patch version, e.g. 3.8.8
• jupyterlab was 2.2.6 => should be specified to be at least version 3 (but not 4 alpha): >=3.0.0,<4.0.0a0

mentioned in issue #48 (closed)

created merge request !39 (closed) to address this issue

mentioned in merge request !39 (closed)

created merge request !40 (closed) to address this issue

mentioned in merge request !40 (closed)

created merge request !46 (closed) to address this issue

mentioned in merge request !46 (closed)

mentioned in merge request !47 (merged)

mentioned in commit 3f8667c3

closed with merge request !47 (merged)

mentioned in commit ab1807cf