Docsy migration followup items - 2021Q1

Work items in follow up to #479:

- [x] Update README with site cloning instructions now that we have submodules #594
- [ ] Duplicate partial inclusion in head.html and head-end.html #775
- [ ] HTML head meta elements are duplicated #745
- [ ] Homepage has empty title/name for opengraph, twitter, and itemprop #744
- [x] Reducing technical debt
- [x] Create and use a grpc-docsy theme to hold docsy file overrides #669
- [x] Npm packages: drop bulma -- it isn't used anymore -- refresh other packages #614
- [x] Drop unused `figure.html` shortcode #678
- [x] Drop style.sass since we aren't using bulma anymore #642
- [x] Config file has dead code from pre-docsy times #655
- [x] Use `assets/scss/_styles_project.scss` to include project-specific styling #700
- [x] SCSS: revert import order of site variables? #668 Fixed via #702
- [x] SCSS: why redefine $gray-100? #703
- [x] SEO:
- [x] Google Search Console is reporting lots of errors #600
- [x] Prevent indexing of the old GitHub-pages site (#599)
- [x] Allow indexing of API docs published from grpc/grpc (#612)
- [x] Regression:
- [x] docsy submodule was regressed to May 2020 #603
- [x] Layout issue with Showcase page #630
- [x] Footer updates are needed #660
- [ ] Cleanup
- [ ] Config: revisit meta tags including those for Twitter and Open Graph #657
- [x] Homepage isn't mobile friendly #689
- [x] Homepage: fix logo and image sizes, they're too big #696
- [x] Homepage: docsy & styles cleanup, slight redesign to reduce oversized element sizes #695
- [x] #582
- [ ] #623
- [x] Upgrade to latest docsy version (2021/02) #610

Questions (not addressed in previous points):

- [ ] Use a shared layout for the About, Showcase and Community pages? #667