docs/concept: the logic of cue

Imported from master branch at commit c06bde0 file:

    /content/en/docs/concepts/logic.md

with only those markup changes that seem to be required, such that
alpha's Hugo will handle the file without complaint.

The presence of several incompatible markup tags in the source document
(e.g. "block/sidebyside") means that a commit with only very minor
(markup) changes simply isn't possible for this page. This CL imports
the file with as small a set of non-text changes as possible, and with
no (intentional) text changes. As such, the diff is not included in this
commit message but can be rendered with:

    diff -wu <(git show c06bde0:content/en/docs/concepts/logic.md) content/docs/concept/the-logic-of-cue/en.md

Making this page preprocessor aware follows in a later CL.

For cue-lang/docs-and-content#75.

Preview-Path: /docs/concept/the-logic-of-cue/
Change-Id: If1a7b895adf249e22f7241c8471ed40d9476a169
Signed-off-by: Jonathan Matthews <[email protected]>
Dispatch-Trailer: {"type":"trybot","CL":1169015,"patchset":3,"ref":"refs/changes/15/1169015/3","targetBranch":"alpha"}