Mention the option to encrypt the root disk on supported architectures

with miod

spelling

Fix a quoting typo in comments

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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