[MISC] general documentation updates

[sappelhoff]

includes:

• adding a dependabot that will keep our GitHub Action versions up to date
• some typo and spelling consistency fixes
• brief intro to the way the changelog is generated
• updating "DECISION MAKING" slightly so that "squash commits" and "rebase commits" of PRs are also valid methods of merging PRs (but before a release, the last PR MUST be a "merge commit")
• replacing @franklin-feingold's email in the Code of Conduct with the general bids-maintainers GMAIL account, cc @bids-standard/maintainers
• updating the "PR tags" that we use (like [ENH], [FIX], ...) with [SCHEMA], which we use a lot now
• add documentation about internal links from schema files and the SPEC_ROOT fix we have introduced recently
• move @franklin-feingold to "past maintainers" in the BIDS maintainers table
• list @rwblair as a bids-maintainers with note "bids-validator maintainer", because in practice there is no difference between a bids-maintainer and a bids-validator maintainer

To Do

• if the change of contact in the Code of Conduct is agreed upon, we need to carry it over to: https://github.com/bids-standard/.github/blob/master/CODE_OF_CONDUCT.md --> Update CoC contact .github#5
  • do the same for "bids-apps": update CoC contact bids-apps/.github#1
• integrate related changes in bids-website: General doc updates bids-website#237

[codecov]

Codecov Report

Merging #1168 (dbf6e16) into master (f20986b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1168   +/-   ##
=======================================
  Coverage   91.60%   91.60%           
=======================================
  Files          10       10           
  Lines        1060     1060           
=======================================
  Hits          971      971           
  Misses         89       89           

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