Add committer/PMC guidelines to the community website

[vinishjail97]

What

Adds a Becoming a Committer page to the community section of the website, documenting the path from contributor to committer and PPMC member.

The page covers:

• Ways to contribute (code, reviews, RFCs, community support, docs, releases)
• Committer criteria — sustained contribution, quality over quantity, reviewing, ownership, community involvement, and conduct
• PPMC member criteria — the governance responsibilities layered on top
• The nomination and voting process, following ASF convention

It also adds a sidebar_position to the existing community page so ordering stays stable.

Why

XTable does not currently document how contributors progress to committer/PPMC. Publishing clear, merit-based guidelines makes the process transparent and fair, and gives contributors a concrete target. This mirrors how established ASF data projects publish theirs (Apache Iceberg, Hudi, Spark).

Notes for reviewers

• Uses PPMC terminology since XTable is incubating (to be updated to PMC at graduation).
• The voting threshold is written as majority vote, open ≥72 hours (matching Hudi and the common ASF default). Happy to switch to consensus if the community prefers.
• Verified with npm run build; the page renders at /community/becoming-a-committer.

🤖 Generated with Claude Code

[zabetak]

LGTM!

One point that contributors usually miss is the following:
"It’s important to remember that becoming a committer is not a reward, or a recognition, so much as that it is the project expressing self-interest." [1]

In other words contributing just code is not always sufficient to make you a committer.
There is no strong incentive from the project perspective to invite you to become a committer since you are not doing something that you cannot do as a contributor.

When you get more involved with the project and people start trusting your reviews and judgement to get patches merged then you become an asset that the project needs. At this point it makes sense for the project to invite you.

Every open-source project is extremely grateful for its contributors but not all of them become committers.

[1] https://community.apache.org/contributors/becomingacommitter.html