Clarify that some of the meetings on the calendar are not open to the public

### Description

Regarding: https://graphql.org/community/upcoming-events/

The page seems to suggest that all meetings on the calendar are open to the public, but that is not the case - some of the meetings on the calendar are Foundation meetings which are not open to the public, they are listed publicly to help WG organizers to not cause scheduling conflicts when scheduling their technical meetings.

> All GraphQL technical meetings are open to anyone who is interested, so long as they first complete the GraphQL Specification Membership agreement.

The various Foundation committees (budget, content, marketing, community/conference, etc) are NOT technical meetings. Technical meetings are the WG meetings: GraphQL Spec, GraphQL-over-HTTP, Composite Schemas, GraphQL.js, Incremental Delivery, Nullability, etc.

> All GraphQL project meetings should appear on the GraphQL public calendar. If a project's meeting isn't on the calendar or is incorrect, please email operations@graphql.org.

The issue here is "GraphQL project meetings" is referring to the technical meetings (meetings that run under the WG), but that's not clear - you could think that Foundation was included in project (it's not).

We could clarify with something like "All GraphQL technical meetings should appear on the GraphQL public calendar. If a project's meeting isn't on the calendar or is incorrect, please email operations@graphql.org. Note that the calendar will also contain entries for private GraphQL Foundation meetings, these are not open to public attendance but are announced to help avoid scheduling conflicts."

> Who can join GraphQL project meetings: Anyone who has signed the free GraphQL Specification Membership agreement.

We should clarify again that this applies to the technical meetings, not to the Foundation meetings such as board, budget, community, conference, marketing etc. E.g. "Who can join GraphQL meetings: Anyone who has signed the free GraphQL Specification Membership agreement may attend GraphQL technical meetings. Other meetings are open to Foundation members and select other parties only."

### Steps to Reproduce

Visit the above linked page.

### Expected Result

Clarity.

### Actual Result

Confusion: why are events on the calendar that I cannot attend? What are "project meetings"?

### Additional Context

I'm not filing a fix for this directly due to #1604.

cc @Urigo @dimaMachina @jorydotcom @leebyron

まず、https://graphql.org/community/upcoming-events/ の文言と、この issue で提案されている明確化を確認してください。公開の技術ミーティングと非公開の GraphQL Foundation ミーティングを区別し、適用される参加ルールを説明するようにページを更新してください。カレンダーとミーティングに関する案内から、すべてのイベントが一般公開されているような印象がなくなれば完了です。

索引モデルが issue の本文から書いたものです。