Add documentation for NWB extensions and publishing process

[bendichter]

• Created a new documentation file for documenting extensions, detailing the process of generating HTML documentation from YAML sources.
• Added examples of real-world NWB extensions, including a detailed example for lab-specific metadata extension.
• Introduced a comprehensive guide on publishing extensions, covering GitHub, PyPI, and NDX Catalog processes.
• Documented the Spec API for creating custom neurodata types, including specifications for Groups, Datasets, Attributes, and Links.
• Added computation times for various example scripts in the gallery.

Motivation

What was the reasoning behind this change? Please explain the changes briefly.

How to test the behavior?

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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 ruff check . && codespell 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

All modified and coverable lines are covered by tests ✅

Project coverage is 94.83%. Comparing base (b331e9c) to head (9802e80).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #2070   +/-   ##
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  Coverage   94.83%   94.83%           
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  Files          28       28           
  Lines        2749     2749           
  Branches      708      708           
=======================================
  Hits         2607     2607           
  Misses         82       82           
  Partials       60       60           

Flag	Coverage Δ
integration	73.99% <ø> (ø)
unit	84.83% <ø> (ø)

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