docs(formula): trim the FAQ to three entries

Drops three FAQs that were filler:
- "Does the formula run on my warehouse?" (answer was obvious)
- "Can I mix aggregates and scalar expressions?" (niche; the function
  reference already implies yes)
- "How do I reference a field with unusual characters?" (edge case
  already covered by the "use SQL" escape-hatch in the intro)

Also simplifies the "I found a bug" FAQ to the standard Slack + GitHub
pattern used in contact/contact-info.mdx, rather than a bespoke copy.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: joaoviana

guides/formula-table-calculations.mdx

@@ -211,30 +211,10 @@ Existing SQL table calculations keep working on every warehouse. The
 beta is strictly additive.
 </Accordion>
 
-<Accordion title="Does the formula run on my warehouse?">
-Yes. Lightdash compiles your formula to SQL using the right syntax
-for your warehouse and runs it alongside the rest of your query.
-There's no separate formula engine or extra round-trip. The generated
-SQL is part of the same query as your metrics and dimensions.
-</Accordion>
-
-<Accordion title="Can I mix aggregates and scalar expressions in the same formula?">
-Yes. When you use `SUM`, `AVG`, `COUNT`, etc. in a formula, we
-compile them to the correct aggregate SQL for your warehouse
-(including the window semantics expected in a table calculation).
-Mixing aggregates and scalar expressions is supported.
-</Accordion>
-
-<Accordion title="How do I reference a field with unusual characters in the name?">
-Use the exact column name shown in the results table header. If your
-field contains characters that aren't valid in a formula identifier,
-use the SQL editor for that specific calculation.
-</Accordion>
-
 <Accordion title="I found a bug or want a function that isn't listed">
-Please let us know. Reach out through your CSM, the Slack community,
-or file a GitHub issue. We're actively growing the function catalog
-through the beta.
+Post in the [Lightdash Community Slack](https://join.slack.com/t/lightdash-community/shared_invite/zt-3pbwqmq5e-vhTc7HHcS787w618ngoPHA)
+or [open a GitHub issue](https://github.com/lightdash/lightdash/issues).
+See [Contact Us](/contact/contact-info) for more options.
 </Accordion>
 
 </AccordionGroup>