PBM-1298 Updated color scheme to differentiate technology

[nastena1606]

Updated logo and favicon Added extra ref to doc homepage

new file: _resource/overrides/partials/header.html new file: docs/_images/mongodb-fav.svg new file: docs/_images/mongodb-mark.svg deleted: docs/_images/percona-favicon.ico deleted: docs/_images/percona-favicon.png deleted: docs/_images/percona-favicon.svg deleted: docs/_images/percona-logo.png deleted: docs/_images/percona-logo.svg modified: docs/css/design.css new file: docs/css/landing.css new file: docs/css/mongodb.css modified: mkdocs-base.yml

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