update Gitter to Discord across ecosystem

[jGaboardi]

This issue will serves as a stub for updating any Gitter mentions/links to Discord here and across the ecosystem.

• pysal/pysal
  • update chat links: gitter >> discord #1297
  • update chat link in CONTRIBUTING.md #1334
• pysal/governance
  • code_of_conduct.rst
    • add wei to CoC -- update discord chat governance#14
• pysal/pysal.github.io – update chat links: gitter >> discord pysal.github.io#220
• pysal/access
• pysal/esda – update chat link -- gitter >> discord esda#325
• pysal/giddy – update chat from gitter to discord giddy#227
• pysal/inequality
• pysal/legendgram
• pysal/libpysal – update chat links: gitter >> discord libpysal#679
• pysal/mapclassify – no current link to gitter badge
• pysal/mgwr
• pysal/momepy – DOC: add links to discord momepy#535
• pysal/pointpats
• pysal/segregation
• pysal/spaghetti – update chat links: gitter >> discord spaghetti#758
• pysal/spglm – discord channel for spglm? spglm#55
• pysal/spint
• pysal/splot
• pysal/spopt – remove link to gitter room & add link to Discord channel spopt#441
• pysal/spreg
• pysal/spvcm archived
• pysal/tobler

[martinfleis]

Also PySAL website?