docs: add GitHub sponsors links, add Nx logo to homepage for sponsorship

[brandonroberts]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngrx/platform/blob/main/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[x] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Closes #

What is the new behavior?

• Nx logo is added to the homepage. Sponsorship links are updated to GitHub sponsors
• Also fixes a bug with serving the docs locally

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

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[timdeschryver]

There are some other links that point to open collective, do we also want to change them?

• projects\ngrx.io\content\marketing\index.html
• .github\ISSUE_TEMPLATE\feature-request.yml
• .github\ISSUE_TEMPLATE\documentation-report.yml
• .github\ISSUE_TEMPLATE\bug-report.yml
• .github\ISSUE_TEMPLATE.md
• CONTRIBUTING.md