Fix typo and update guidance to point to dfeR package

[cjrace]

Overview of changes

Update references from the Analysts' Guide to point to dfeR for connecting to SQL.

Why are these changes being made?

We have a function in dfeR that we will maintain there, as well as a vignette that walks through connecting to SQL.

Detailed description of changes

As it says, plus:

• Removed link to old C+C materials that just shows a blank page
• Removed random HTML files that popped up in my Git changes, assuming they shouldn't have been there to start with

Issue ticket number/s and link

None.

Checklist before requesting a review

• I have checked the contributing guidelines
• I have checked for and linked any relevant issues that this may resolve
• I have checked that these changes build locally
• I understand that if merged into main, these changes will be publicly available

[cjrace]

Add SQL functions into dfeR

[cjrace]

Update analysts guide connecting to SQL to point to dfeR get_clean_sql()