Install on linux instructions out of date

[ianhi]

The instructions in https://micro-manager.org/Linux_installation_from_source_(Ubuntu) have links to the the wrong places and may be out of date in other ways. It would be good to either update these or to redirect to https://github.com/micro-manager/micro-manager/blob/main/doc/how-to-build.md which seems to be more up to date

[ianhi]

I would also suggest deleting: https://micro-manager.org/Linux_installation_from_source_(Gentoo) as the repo it links to hasn't been updated in 5 years

[marktsuchida]

Agreed. how-to-build.md also needs some updates (will do), but it makes sense to keep detailed build instructions in the code repository rather than the wiki. I'm also planning to write VS2019 instructions for Windows asap.

[marktsuchida]

I've updated the main links, and how-to-build.md has also been updated.

The rest involves deleting out-of-date pages (preserving anything not covered elsewhere), pending decision on #94 (comment).