Sage/a guide to building decentralized loan dapp (celo-org#748)

* first draft

* first draft

* first draft

* second draft

* third draft

* update: added metadata

* update: fixed typos

* update: added header image

* update: removed header before introduction

* update

* building decentralized investment platform

* removed duplicates'

* corrections

* draft

* updated

* typos

* typos

* typos

* Fixed MARKDOWN

* added celosage tag

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Co-authored-by: developerkunal <[email protected]>