Dbt Docs, Use user provided index.html

[ismailsimsek]

Resolves dbt-labs/dbt-docs#50

Problem

Currently its not possible to customize index.html file when generating dbt docs. It will be nice feature to be able to change its CSS and HTML content.

Solution

with this change if any of the dbt-docs folders containing index.html then this will be used, instead of dbt provided default file.

Checklist

• I have read the contributing guide and understand what's expected of me.
• I have run this code in development, and it appears to resolve the stated issue.
• This PR includes tests, or tests are not required or relevant for this PR.
• This PR has no interface changes (e.g., macros, CLI, logs, JSON artifacts, config files, adapter interface, etc.) or this PR has already received feedback and approval from Product or DX.
• This PR includes type annotations for new and modified functions.

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[ismailsimsek]

@drewbanin @dbeatty10 @jtcohen6 what do you think of this solution? if it looks good i will work on it further..