[MAINTENANCE] Replace Slack links with Discord links.

[alexsherstinsky]

Scope

• All Slack links are replaced with links referring to the "Predibase / LoRAX / Ludwig" community on Discord.
• There is a sibling documentation pull request.

Code Pull Requests

Please provide the following:

• a clear explanation of what your code does
• if applicable, a reference to an issue
• a reproducible test for your PR (code, config and data sample)

Documentation Pull Requests

Note that the documentation HTML files are in docs/ while the Markdown sources are in mkdocs/docs.

If you are proposing a modification to the documentation you should change only the Markdown files.

api.md is automatically generated from the docstrings in the code, so if you want to change something in that file, first modify ludwig/api.py docstring, then run mkdocs/code_docs_autogen.py, which will create mkdocs/docs/api.md .

[github-actions]

Unit Test Results

  6 files  ±0    6 suites  ±0   14m 58s ⏱️ +33s
12 tests ±0    9 ✔️ ±0    3 💤 ±0  0 ❌ ±0 
60 runs  ±0  42 ✔️ ±0  18 💤 ±0  0 ❌ ±0 

Results for commit e8dc27f. ± Comparison against base commit f28a3c5.

♻️ This comment has been updated with latest results.