Docs: Add remote execution architecture documentation.

Description

Add documentation for the remote execution architecture in BuildStream.

Proposed changes

Changes proposed in this merge request:

• Add a description of the remote execution architecture
• Add ODG diagram for remote execution and the SVG exported from that

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marked as a Work In Progress

changed the description

added 96 commits

• 63f0df0d...3f663d82 - 95 commits from branch master
• 33438248 - Docs: Add remote execution architecture documentation.

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I found this a very clear and informative read. Nice work @jmacarthur :)

approved this merge request

resolved all discussions

unmarked as a Work In Progress

added 31 commits

• 33438248...abef70fe - 30 commits from branch master
• 5c208698 - Docs: Add remote execution architecture documentation.

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merged

mentioned in commit 5af455a4