feat: support links in sidebar

[kopecs]

PR checklist:

• Purpose of the code is evident to future readers
• Tests included or PR comment includes a reproducible test plan
• Documentation is up-to-date
• A changelog entry was for any user-facing change
• Change has no security implications (otherwise, ping security team)

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Adds a new section in the sidebar with links to join slack, view documentation, or email support:

[kopecs]

Rebasing to get updated test list

[ajbt200128]

Looks good, we probably want to rename the sidebar to just Semgrep:

semgrep-vscode/package.json

Line 215 in bca80e3

"title": "Semgrep Search",

[kopecs]

Ah true.