Reorganize nwb-overview to here

[rly]

• Intro to NWB -> Get Started

• Conversion tutorial -> Get Started

• Reading NWB Files -> Get Started. Add page that points to reading tutorial. Link to DANDI as well.

• Extending NWB -> PyNWB docs at top-level (ideal place is catalog but pynwb is more accessible). Add a high-level overview on nwb.org.

• Various Tools top-level pages -> redirects

• Community Gallery -> Community

• FAQ -> Resources

• Accessing NWB Sources -> delete

• NWB Software Analytics -> move to a separate nwb-project-analytics.readthedocs.io because deployment will be challenging and nwb-overview is not an ideal title