Update the index of the documentation

[oruebel]

Motivation

Fix #1401 This PR enhances the index of the documentation and cleans up numerous issues with regard to duplicated and outdated items.

• Update the main page index of the documentation to make it easier to navigate by creating sections focused on the main readers: I.e., A Getting Started section for users, a Resources section for further reading, a For Developers section for everything related to development, and a Contributing section with all legal items.
• Split installation instructions into user and developer sections in the appropriate chapters (similar to HDMF)
• Remove duplicate overview_intro.rstwhich included only basic text already discussed on the main landing page
• Merged the overview_nwbfile.rst with the NWB basics tutorial
• Cleaned up the Resources section to move developer-focused items to the For Developers section
• Moved the Tutorials to the Getting Started chapter to make them easier to find
• Moved the developer FAQ from "CONTRIBUTING.rst" to the developer installation instructions

Remaining issues

• Update the outdated overview_software_architecture.rst and refer to the HDMF docs for details. Created Update software architecure docs #1429 to document this issue to be addressed in a separate PR.

Checklist

• Did you update CHANGELOG.md with your changes?
• Have you checked our Contributing document?
• Have you ensured the PR clearly describes the problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running flake8 from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.

[codecov]

Codecov Report

Merging #1402 (0aa813d) into dev (e484bfd) will decrease coverage by 0.04%.
The diff coverage is n/a.

❗ Current head 0aa813d differs from pull request most recent head c235e4c. Consider uploading reports for the commit c235e4c to get more accurate results

@@            Coverage Diff             @@
##              dev    #1402      +/-   ##
==========================================
- Coverage   77.31%   77.26%   -0.05%     
==========================================
  Files          37       37              
  Lines        2715     2714       -1     
  Branches      450      450              
==========================================
- Hits         2099     2097       -2     
  Misses        535      535              
- Partials       81       82       +1     

Impacted Files	Coverage Δ
src/pynwb/icephys.py	89.52% <0.00%> (-0.35%)	⬇️

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[rly]

@oruebel is this ready to merge? If so, please Squash and Merge.

[oruebel]

@rly can you please re-approve. I had to resolve minor merge conflicts in the CHANGELOG and a heading of a tutorial in order to catch the PR up with dev, so that your review was dismissed. Otherwise, this is good to merge. Feel free to squash and merge.