Update documentation and remove specific software packages and internal data models

[kzollove]

This PR:

• removes gaiaCore and gaiaDB, which now both have their own separate repositories
• Adds DDL-generating scripts (and DDLs) for LOCATION_HISTORY and EXTERNAL_EXPOSURE
• Simplifies documentation website structure
• Fixes:
  • Revise OHDSI GIS Workgroup's mission in the GitHub documentation #377
  • Update Roadmap section of GitHub documentation #378
  • Revise the Goals section in GitHub documentation #379
  • Replace Problem space section of Github documentation with GIS Toolchain section #380
  • Replace Meeting Schedule section of Github documentation with Collaboration Approach section #381
  • Update/correct list of linked presentations/publications to the Github documentation #382
  • Add figure and description to the Toolchain description in Github documentation #383

[kzollove]

@jshoughtaling @tibbben @rtmill

Wanted to make you all aware of this major change which strips out gaiaCore and gaiaDB into their own repositories. I will merge this ASAP. Documentation site is still WIP after this merge.