Repo links improvements

[smainz]

This refactors the repo links, so it is easier to add new types of repositories.

Current github_* parameters have been deprecated and replaced by these params:

# Repository configuration (URLs for in-page links to opening issues and suggesting changes)
[params.repo]
  # The type of forge (allowed values github, azure, gitea)
  # If you enter a different type, you can add a layout/partials/repo-links-<your_type>.html
  # template for rendering links.
  type = "github"                                      
  url = "https://github.com/google/docsy-example" 
  # Uncomment this if your repo does not have "main" as the default branch,
  # or specify a new value if you want to reference another branch in your git links
  branch = "master"                                  
  # Specify a value here if your content directory is not in your repo's root directory
  #subdir = "docs"                                     
  # An optional link to a related project repo. For example, the sibling repository where your product code lives.
  project_url = "https://github.com/google/docsy"

Scope of this PR is:

• refactor git repo configuration
• deprecate the current github_* cofiguration (still works, but gives a warning)
• add support for gitea / forgjo git repos
• add support for Azure DevOps repos
• add support for gitlab repos
• add support for custom repo type
• update documentation

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[smainz]

This is still work in progress, but can be finalized soon by updating the docs and possibly taking the azure repo links from #1310. @chalin Does this go into the right direction, so time permits for Release .10.0 (#1812)?

[chalin]

Thanks @smainz. While #1841 is on the radar, I don't think that it will make 0.10.0 -- the release has been delayed much longer than I hoped for already. I'll be working on #1841 and taking a look at this PR as soon as I can, but it might not be before 0.12.0 as the way things are shaping up atm 🤷🏼‍♂️.

[smainz]

I can try to unify

• [x] #355,
• [x] #375 and #1000
• [x] #794 and #1310
• ~~[ ] #1859~~ (no URLs provided)
• ~~[ ] and maybe #256~~ (use type: custom)

into one PR

if the authors agree and provide the relevant urls.