update install.md path

update list file paths

Mention the option to encrypt the root disk on supported architectures

with miod

spelling

Update the paths for install.md and the list files.

OK sthen@

Build and document bsd.mp on alpha.

Try to catch up with installer changes in the installation notes.
Also, the per-platform features are moved to a common place, which makes
the MD layout easier to work on. m4.common is still a tangle

Try to catch up with installer changes in the installation notes.
Also, the per-platform features are moved to a common place, which makes
the MD layout easier to work on. m4.common is still a tangled mess of subtly
placed dnl, though...

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