Refactor the documentation structure

[pleku]

Fixes #1698

Moves all files around to make sure content is aligned into preferable reading order.
Also adds a very shallow introduction / architecture page.

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This change is 

[pleku]

Review status: 0 of 92 files reviewed at latest revision, 1 unresolved discussion.

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a discussion (no related file):
Still need to revise and fix the page ordering and headers for all sections (used in vcom menu).

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Comments from Reviewable

[denis-anisimov]

Reviewed 159 of 159 files at r1.
Review status: all files reviewed at latest revision, 2 unresolved discussions.

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flow-documentation/introduction/introduction-overview.asciidoc, line 41 at r1 (raw file):

// CODE SNIPPET //

Is it supposed to be filled later on?
Is it OK to keep it as is for now?

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Comments from Reviewable

[pleku]

Review status: 70 of 93 files reviewed at latest revision, 2 unresolved discussions.

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flow-documentation/introduction/introduction-overview.asciidoc, line 41 at r1 (raw file):

Previously, denis-anisimov (Denis) wrote…

// CODE SNIPPET //

Is it supposed to be filled later on?
Is it OK to keep it as is for now?

Yeah I have separate list of things to fix in the documentation content, this one is there and I just with to get the structure change now in so that I don't have to keep on fixing merge conflicts...

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Comments from Reviewable

[pleku]

Review status: 70 of 93 files reviewed at latest revision, 1 unresolved discussion.

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a discussion (no related file):

Previously, pleku (Pekka Hyvönen) wrote…

Still need to revise and fix the page ordering and headers for all sections (used in vcom menu).

Done

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Comments from Reviewable

[denis-anisimov]

Reviewed 13 of 159 files at r1, 22 of 26 files at r2.
Review status: all files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable