MAINTAINERS: add Manu Gupta (manugupt1) as a REVIEWER

[AkihiroSuda]

Manu Gupta (@manugupt1) has been very actively contributing to the project:
https://github.com/containerd/nerdctl/pulls?q=author%3Amanugupt1+

So I'd like to invite @manugupt1 as a reviewer.

Needs explicit LGTM from @manugupt1 and 1/3 of the nerdctl Committers ( $ceil \left( 1 \times \frac{2}{3} \right) = 1$ ), according to https://github.com/containerd/project/blob/main/GOVERNANCE.md :

After a candidate has been informally proposed in the maintainers forum, the
existing maintainers are given seven days to discuss the candidate, raise
objections and show their support. Formal voting takes place on a pull request
that adds the contributor to the MAINTAINERS file. Candidates must be approved
by 2/3 of the current committers by adding their approval or LGTM to the pull
request. The reviewer role has the same process but only requires 1/3 of current
committers.

If a candidate is approved, they will be invited to add their own LGTM or
approval to the pull request to acknowledge their agreement. A committer will
verify the numbers of votes that have been received and the allotted seven days
have passed, then merge the pull request and invite the contributor to the
organization.

For non-core sub-projects, only committers of the repository that the candidate
is proposed for are given votes.

• @manugupt1
• @ktock (nerdctl Committer)
• @fahedouch (nerdctl Committer)

I'd also like to get a few LGTMs from the Core Committers too. (not necessary)

This PR will remain open for 7 days.

[fahedouch]

LGTM

[manugupt1]

Thanks for inviting me.LGTM

[ktock]

LGTM

[fuweid]

LGTM

[Zheaoli]

LGTM! Welcome!

[AkihiroSuda]

Welcome @manugupt1