docs: add page for Hexagate monitoring program and add Hexagate to base-services-hub table

[tomryan-cb]

What changed? Why?
Adding documentation to base/docs on Base's monitoring program with Hexagate.

• Added a new page hexagate-monitoring.md describing Hexagate’s free Ecosystem Tier offering for apps and protocols builders on Base.
• Added Hexagate as a new security service to the table in docs/pages/base-services-hub.mdx.
• Updated the sidebar (sidebar.ts) to include "Smart contract monitoring with Hexagate" under the Security section.

Notes to reviewers

• More details on the Hexagate monitoring program for Base builders can be found in these Base and Hexagate articles.
• This program is mentioned on launchonbase.xyz.

How has it been tested?
Yes, ran locally. See screenshots.

Have you tested the following pages?

BaseWeb

• [] base.org
• [] base.org/names
• [] base.org/builders
• [] base.org/ecosystem
• [] base.org/name/jesse
• [] base.org/manage-names
• [] base.org/resources

BaseDocs

• docs.base.org
• docs sub-pages

[vercel]

@tomryan-cb is attempting to deploy a commit to the Coinbase Team on Vercel.

A member of the Team first needs to authorize it.

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/2	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 1 Sum 2