source/index.rst: Use the README as our intro

So we keep the contents of the README and https://buildstream.gitlab.io/buildstream/ in sync

(I was about to add the coverage badge in https://buildstream.gitlab.io/buildstream/ but I think this is better, even more after !403 (merged) is merged)

mentioned in merge request !403 (merged)

restored source branch jjardon/doc_readme

changed the description

added Documentation label

added 7 commits

• 3ffc94a3...226880ee - 6 commits from branch master
• 882f756c - source/index.rst: Use the README as our intro

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marked as a Work In Progress

added 34 commits

• 882f756c...ae1d5093 - 31 commits from branch master
• b5495364 - Add about page
• c1c64ee6 - Fix
• 63ab0297 - doc/source/index.rst: Replace welcome paragraph

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added 1 commit

• d50e9e60 - doc/source/index.rst: Replace welcome paragraph

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added 1 commit

• 28def878 - doc/source/index.rst: Replace welcome paragraph

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unmarked as a Work In Progress

added 4 commits

• 28def878...a850b8ed - 2 commits from branch master
• cd75869f - Add about page
• 899c3ce0 - doc/source/index.rst: Replace welcome paragraph

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merged