Fix spec links and shorten line lengths

[duckontheweb]

Description:

• Fixes GitHub username in docs/conf.py
• Uses the :stac-spec: extlink for all links to the STAC spec. This will automatically generate links to the version of the spec from pystac.version.STACVersion.DEFAULT_STAC_VERSION (there were a number of these links that were still pointing to old spec versions).
• Uses the :stac-ext: extlink for all links to STAC Extensions specs.
• Updates RST files to have a max line length of 88 to match linting on other files
• Uses doc8 to lint all source files for the docs (except tutorials) as part of ./scripts/test
  If this last one seems too heavy-handed I'm happy to remove it, but I thought it would help keep us from getting some of the really long lines that had snuck in before.

PR Checklist:

• Code is formatted (run scripts/format)
• Tests pass (run scripts/test)
• This PR maintains or improves overall codebase code coverage.
• Changes are added to the CHANGELOG. See the docs for information about adding to the changelog.

[codecov-commenter]

Codecov Report

Merging #368 (984b2e9) into main (e90f809) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #368   +/-   ##
=======================================
  Coverage   89.77%   89.77%           
=======================================
  Files          37       37           
  Lines        4677     4677           
=======================================
  Hits         4199     4199           
  Misses        478      478           

Impacted Files	Coverage Δ
pystac/catalog.py	93.51% <ø> (ø)
pystac/extensions/label.py	85.89% <ø> (ø)
pystac/layout.py	92.44% <ø> (ø)
pystac/stac_object.py	95.08% <ø> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e90f809...984b2e9. Read the comment docs.